ZSM-5 catalysts

Applied Catalysis A: General 262 (2004) 155–166

Simultaneous 1-pentene hydroisomerisation and thiophene hydrodesulphurisation over sulphided Ni/FAU and Ni/ZSM-5 catalysts B. Pawelec, R. Mariscal, R.M. Navarro, J.M. Campos-Martin, J.L.G. Fierro∗ Instituto de Catálisis y Petroleoqu´ımica, CSIC, c/Marie Curie, Campus UAM, s/n Cantoblanco, E-28049 Madrid, Spain Received in revised form 24 November 2003; accepted 24 November 2003

Abstract Bifunctional catalysts of nickel sulphide supported on zeolites NaY, USY and ZSM-5 have been tested in simultaneous thiophene hydrodesulphurisation (HDS) and 1-pentene hydroisomerisation (HYD/ISO). The catalysts were prepared by ion-exchange and studied in their oxidic and sulphided forms using several physico-chemical techniques such as chemical analysis, X-ray diffraction (XRD), IR of the framework vibration, N2 adsorption–desorption isotherms, ammonia temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) of adsorbed NO. The best balance of the HDS and HYD functions was achieved over the Ni(6.6)USY sample, which showed an HDS/HYD ratio about three-fold larger than a commercial CoMo/Al2 O3 catalyst as well as some skeletal isomerisation of 1-pentene (selectivity 2.7%). On the basis of the catalyst structure–activity relationship, it can be inferred that the catalytic response of Ni(6.6)USY catalyst is linked to a homogeneous distribution of nickel species within the zeolite pores and also to the greater acidity of the USY zeolite. NaY-supported and Ni(2.7)USY catalysts showed almost exclusively double bond isomerisation whereas both double bond and skeletal isomerisation were observed for the ZSM-5 and its nickel-loaded counterparts. Factors minimizing 1-pentene hydrogenation during the simultaneous HDS of thiophene and 1-pentene hydroisomerisation over sulphided Ni/NaY, Ni/USY and Ni/ZSM-5 catalysts are discussed. © 2003 Elsevier B.V. All rights reserved. Keywords: Ni-sulphide; ZSM-5; USY; NaY; Hydroisomerisation; 1-Pentene; Thiophene; Hydrodesulphurisation

1. Introduction The new environmental regulations planned for 2005 by the EU (Directive 98/70/EC) consider the reduction of the sulphur level in the gasoline pool down to 50 ppm [1]. Since nearly all the sulphur (85–95%) in the gasoline pool comes from the fluid catalytic cracking (FCC) unit, this gasoline fraction has been the focus of research activities in the last few years. Alternatively, the reduction of the S content in the gasoline pool could be achieved by: (i) pretreating the FCC feed; (ii) removal of S-compounds during the FCC process by cracking or selective adsorption; or (iii) hydrotreating FCC gasoline. From the economic point of view, the latter solution is cheaper than the former because it requires a lower amount of H2 . Deep hydrodesulphurisation of the ∗ Corresponding author. Tel.: +34-91-585-4760; fax: +34-91-585-4769. E-mail address: [email protected] (J.L.G. Fierro). URL: http://www.icp.csic.es/eac/index.htm.

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.11.037

FCC gasoline fraction is usually accomplished by the use of conventional CoMo or NiMo catalysts. However, as the degree of desulphurisation increases, the research octane number (RON) of gasoline products decreases due to the hydrogenation of olefins as well as of aromatics present in the feed [2–4]. Thus, the challenge will be to tailor new selective catalysts able to remove sulphur while at the same time minimizing the hydrogenation of olefins and aromatics. Many studies have been carried out with the aim of developing selective catalysts to remove sulphur but minimizing the hydrogenation of olefins and octane reduction [5–18]. Most studies suggest that this can be achieved by modification of the catalytic properties, employing a basic support material. Thus, the use of hydrotalcite MgO–Al2 O3 materials [5–7] or basic supports such as MgO [8–13] and alkali-promoted supports have been proposed [14–16]. Some sulphur removal techniques that avoid olefin hydrogenation are based on improving HDS activity by using additives such as USY/ZnO/Al2 O3 [17], which may crack the sulphur com-

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pounds in the gasoline range. Such additives are combined in specifically developed FCC catalysts [18–21]. However, maximum sulphur reduction is only about 40% as compared to the sulphur content of gasoline produced without any additive [18,9]. To date, the most effective way to suppress olefin hydrogenation without reducing HDS function is to modify the promoter substrate, as done in the SCAN-Fining process [22]. Due to the nature of the reactions concerned in the olefins HYD and HDS of S-compounds, bifunctional catalysts with both hydrogenation and acid functions are needed. Olefin hydrogenation requires metallic centers, while the isomerisation through classical or non-classical carbenium chemistry takes place on acidic centers [23]. Concerning the active phases needed for HDS activity, it is generally accepted that the catalytic behavior of sulphided Ni is controlled by the coordinative unsaturation of the Ni ion [24]. The Ni3 S2 phase alone, with the Ni ion coordinated by four S ions, possesses the coordinative unsaturation of Ni ions needed for the HDS reaction [24]. The proposed mechanism of thiophene HDS under high hydrogen pressure involves the perpendicular adsorption of thiophene through the S atom on an S anion vacancy, followed by attack of the S–C bond by H from adjacent OH groups, forming butadiene as the primary product [25]. It is fairly well established that active sites for HDS and olefin HYD are distinct [24,26]. Since olefin HYD has been shown to be highly structure-sensitive [27], the enhancement of HDS/HYD selectivity is expected to be achieved by tailoring the morphology of the active phases at the high-specific area supports. Candidates for the enhancement of HDS/HYD selectivity could be transition metal sulphides deposited on zeolites such as faujasite-type ultrastable HY (USY) and NaY zeolites with three-dimensional pore structure of tetrahedrally connected sodalite cages and the pore size determined by the 12-ring of oxygen atoms [28]. FAU-type zeolite structure comprises hexagonal prisms, sodalite cages, and supercages with maximum entrances of 0.26, 0.26 and 0.74 nm, respectively. This structure makes them suitable for the HDS reaction of large thiophenic molecules [29]. However, because of their pore systems they are expected to be much less suitable for skeletal isomerisation of alkenes than zeolites with a 10-membered ring and a pore diameter between 0.4 and 0.55 nm, such as ZSM-5 [30–32]. Since USY zeolites are obtained by substitution of the Na+ cation of the NaY zeolite by an H+ ion, this zeolite exhibits greater acidity and, therefore, a greater deactivation rate. Taking into account that olefin chain-branching contributes to high octane numbers, research activities are also directed toward improving the isomerisation selectivity of conventional hydrotreating catalysts by the use of medium-pore zeolites such as ZSM-5 [23,30–32]. Ni/ZSM-5 zeolites activated by reduction and sulphidation have been found to be effective for hydroisomerisation of higher alkanes [23,30–32] and thiophene HDS [29], re-

spectively. This is because of the unique two-dimensional pore system of the ZSM-5 zeolite, which comprises interconnecting straight (0.56 nm × 0.53 nm) and sinusoidal channels (0.51 nm × 0.55 nm) [28], which allow a “molecular traffic mechanism” [33]. Recently, Yin et al. [34] found that cracked naphtha products hydrotreated on sulphided Ni/HZSM-5 have relatively higher aromatics and olefin contents (higher RON) than those obtained on CoMo/Al2 O3 catalysts. Nevertheless, to our knowledge no studies addressing simultaneous n-olefin HYD and thiophene HDS over sulphided Ni/FAU and Ni/ZSM-5 catalysts have been reported. Within the above scope, this work was undertaken with a view to comparing the catalytic behaviour of sulphided large pore Ni/NaY and Ni/USY zeolites with medium-pore Ni/ZSM-5 catalysts in the simultaneous thiophene HDS and 1-pentene HYD/ISO. Information regarding the structure of the nickel catalysts, in their oxidized and sulphided forms, was gained by using several physico-chemical techniques, such as chemical analysis, X-ray diffraction (XRD), IR of the framework vibration, N2 adsorption–desorption measurements, ammonia temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) of adsorbed NO.

2. Experimental 2.1. Catalyst preparation Supported nickel-zeolite catalysts were prepared by ion-exchange from ultrastable HY (USY; Conteka, B.V., Sweden), NaY (LZ-Y52 Union Carbide) and ZSM-5 zeolites (Akzo Nobel, Sweden). The characteristics of the zeolites were as follows: USY (Si/Al = 4.56, Na2 O content 0.14 wt.% and unit cell 2.454 nm, BET = 662 m2 g−1 ), NaY (Si/Al = 2.85, Na2 O/Al2 O3 = 0.97, BET = 657 m2 g−1 ) and ZSM-5 (Si/Al = 19, BET = 331 m2 g−1 ). A commercial CoMo/alumina catalyst (2.7 wt.% of Co and 9.5 wt.% of Mo; BET = 212 m2 g−1 ) was used as reference. The zeolite was added to an aqueous solution of Ni(NO3 )2 ·6H2 O (Merck, reagent grade) of concentration of 0.031 and 0.074 mol l−1 calculated to obtain 3.5 and 8 wt.% of Ni nominal content in the catalysts, respectively (a solution-to-zeolite volume ratio of 20:1). The ion-exchange was achieved under constant stirring at 333 K for 16 h. Following this, the suspension was filtered and then thoroughly washed with deionized water in order to remove occluded salt. The ion-exchanged samples were air-dried at 383 K for 24 h, and then calcined in air at 723 K for 2 h. Henceforth, the catalysts will be referred to as Ni(x)NaY, Ni(x)USY, Ni(x)ZSM-5, where x denotes the nickel content (wt.%) determined by chemical analysis. The particular catalyst series will be referred to as Ni/FAU (Ni/USY, Ni/NaY) and Ni/ZSM-5.

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2.2. Characterization of catalysts 2.2.1. Chemical analysis Chemical analysis of the calcined catalysts was performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Perkin-Elmer Optima 3300DV instrument. Prior to analysis, the samples were solubilized in a mixture of HF, HCl and HNO3 at 363 K and homogenized in a microwave oven, after which aliquots of the solution were diluted to 50 ml using Milli-Q deionized water. 2.2.2. X-ray powder diffraction (XRD) The XRD diffraction patterns of the calcined catalysts were obtained according to the step-scanning procedure (step size 0.04◦ , 1 s) with a computerized Seifert 3000 XRD diffractometer, using Cu K␣ (λ = 0.15406 nm) radiation and a PW 2200 Bragg-Brentano θ/2θ goniometer equipped with a bent graphite monochromator and an automatic slit. Scanning 2θ angles ranging from 10 to 70◦ , the XRD line-broadening measurements were carried out using the NiO peak (37.34 or 43.35 in 2θ). The width at half-maximum (fwhm) of these peaks was corrected for instrumental broadening (b). Relative crystallinity was estimated by comparison of the peak intensities for d values of 0.376, 0.329 and 0.290 nm of the catalysts to those of the blank zeolites. 2.2.3. UV-Vis diffuse reflectance spectra (DRS) The electronic spectra of the finely ground calcined samples were recorded in the 240–800 nm range with a Shimadzu UV-2100 spectrophotometer, using BaSO4 as a reference and converted to the Schuster–Kubelka–Munk function. 2.2.4. Nitrogen adsorption–desorption The textural properties of the calcined catalysts were evaluated from the N2 adsorption–desorption isotherms obtained at 77 K over the whole range of relative pressures, using a Micromeritics ASAP-2000 apparatus, for samples previously evacuated at 623 K for 18 h. A value of 0.162 nm2 was taken for the cross-section of the physically adsorbed N2 molecule. BET specific areas were computed from these isotherms by applying the BET method over the 0.005–0.25 P/P0 range. In all cases, correlation coefficients above 0.999 were obtained. 2.2.5. Temperature-programmed reduction (TPR) TPR profiles were obtained on a semiautomatic Micromeritics TPD/TPR 2900 apparatus interfaced with a computer. Prior to the measurements, the catalyst (ca. 50 mg) was dried in a TPR cell at 523 K for 2 h in a stream of He to remove water. Then, the TPR profiles were obtained by passing a 10% H2 /Ar (Air Liquide) flow (60 ml min−1 ) through the sample at temperatures from 303 to 1173 K. Temperature was increased at a rate of 15 K min−1 and the

157

amount of H2 consumed was determined with a thermal conductivity detector (TCD). A cooling trap placed between the sample and TCD was used to retain the water produced during the reduction process. 2.2.6. Temperature-programmed desorption (TPD) of ammonia Ammonia-TPD measurements of the pre-sulphided catalysts were carried out with the same apparatus described for TPR. Before ammonia saturation, the sample (500 mg) was pretreated under a helium atmosphere at 523 K for 1 h, and then sulphided, employing a 10:1 (v/v) H2 /H2 S mixture at 673 K for 2 h. The sample was then cooled to 423 K and ammonia-saturated in a stream of 5% NH3 /He (Air Liquide) (flow, 50 ml min−1 ) for 1 h. After sample equilibration in a flow of He at 423 K for 0.5 h, ammonia was desorbed at a heating rate of 10 K min−1 . The water evolved was trapped in a KOH trap located just before the TCD. The total acidity of the catalyst was obtained by integrating the area under the desorption curve. A semiquantitative comparison of strength distribution was achieved by Gaussian deconvolution of the peaks. 2.2.7. Fourier-transform infrared spectroscopy The zeolite framework vibrations (1400–400 cm−1 ) of the calcined catalysts were recorded at room temperature with a Nicolet 510-FTIR spectrophotometer (at a resolution of 4 cm−1 and averaged over 100 scans). Discs of the zeolite diluted in KBr (2:100 (w/w)) were prepared by pressing the powder (5 T cm−2 ) in a hydraulic die. The IR spectra of chemisorbed NO were recorded on the sulphided samples. The sulphiding procedure entailed heating in flowing helium at 673 K for 0.5 h, exposing the samples to a H2 S:H2 mixture (ratio 1:10) for 2 h at the same temperature, followed by purging in flowing He at 673 K for 15 min and finally cooling to room temperature. All spectra were obtained upon adsorption of 30 mbar NO (1 mbar = 102 Pa) at room temperature, followed by evacuation for 10 min. Net infrared spectra were obtained by subtracting the sample background from the whole spectrum. 2.2.8. X-ray photoelectron spectroscopy (XPS) XP spectra of the presulphided (673 K for 2 h) and used catalysts were recorded on a VG Escalab 200R electron spectrometer equipped with a hemispherical electron analyzer, using a Mg K␣ (hν = 1253.6 eV, 1 eV = 1.603 × 10−19 J) X-ray source. The ECLIPSE software was used to record and analyze spectra. The procedure followed to measure binding energies (BE) and relative proportion of catalyst constituents has been described elsewhere [35]. The Ni 2p3/2 , S 2p, Si 2p and Al 2p core-level spectra were recorded and the corresponding binding energy was referenced to the C 1s line at 284.9 eV (accuracy within ±0.1 eV). Atomic concentrations were determined from integrated peak areas normalized by atomic sensitivity factors [36].

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2.2.9. Activity measurements Activity tests were performed in a high-pressure laboratory set-up equipped with fixed-bed reactor (9.5 mm i.d. and 130 mm length) using fixed experimental conditions. A model feed consisting of thiophene (1000 ppm of S) and 1-pentene (30 wt.%) dissolved in n-hexane was used with the aim of simultaneously monitoring the HDS, HYD and isomerisation functions of the catalysts. For activity tests, 0.5 g of catalyst (0.25–0.30 mm particle size range) was dried in an N2 flow at 623 K for 2 h. Catalyst sulphidation was performed in a 10:1 (v/v) H2 /H2 S mixture, raising the temperature at a rate of 3 K min−1 to 673 K and then performing isothermal sulphidation at this temperature for 2 h. After sulphidation, the oven was cooled to the desired reaction temperature and the reagent/H2 flow was passed over the catalyst. Thermodynamic calculations were made with the aim to see if there are conditions at which desulphurization is maximized while minimizing the olefin hydrogenation. From these calculations, it was shown that both reactions (HDS and HYD) will proceed in the 473–673 K range as G is negative over the entire temperature range. Operation at a certain minimum temperature is necessary to obtain the desired sulphur reduction. Since, both reactions are favored by pressure; it is desirable to increase pressure rather than temperature to achieve the optimal HDS/HYD ratio. Thus, with the aim to favor HDS/HYD ratio and skeletal isomerisation of 1-pentene [37], the temperature was fixed at 523 K and total hydrogen pressure of 10 bar was selected. The other reaction conditions employed were: H2 /hydrocarbon = 100, WHSV = 10 h−1 , the molar flow rate of the 1-pentene and thiophene were 27.3 and 0.3 mmol h−1 , respectively. The CoMo reference catalyst was evaluated under the same conditions than that used in the nickel catalysts. The gas mixture from the reactor was analyzed on-line by FID-GC (Varian 3400 CX). Traces of thiols, which might derive either from the recombination of H2 S with the 1-pentene or incomplete desulphurization, were observed. Besides unreacted model feed compounds (thiophene and 1-pentene), the main products detected were

n-butane, cis- and trans-2-butene, cis- and trans-2-pentene, 2-methyl-2-butene (2M2B), n-pentane (nP), iso-pentane (iP), 3-methyl-1-butene (3M1B) and 2-methyl-1-butene (2M1B). Calculation of 1-pentene isomers was performed following reference [38]. Skeletal isomerisation was measured according to 2-methyl-2-butene selectivity in the pentene fraction (selectivity (S) to branched olefins). Activities were described in terms of HDS of thiophene and HYD of 1-pentene using a specific reaction rate according to the equation: ri =

[Xi · Fi ] mcat

where ri is the specific rate (mmol g−1 h−1 ); Xi the conversion of reactant i (i = thiophene or 1-pentene); Fi the molar flow rate of the reactant i (mmol h−1 ), and mcat is the catalyst weight (g). For HDS reaction, activity decay was expressed as percentage of catalyst deactivation calculated from the ratio [(Xt=2 h − Xt=7 h )/(Xt=2 h )], where X is the thiophene conversion.

3. Results and discussion 3.1. Characterization of calcined catalysts 3.1.1. Chemical composition and X-ray diffraction The chemical compositions of the calcined catalysts, as determined by ICP-AES, are summarized in Table 1. For low Ni-content samples, the elemental analysis confirmed a difference in Ni-loading between the NaY and USY zeolites (3.3 versus 2.7 Ni wt.%), originated by the lower ion-exchange capacity of the USY zeolite as compared to NaY (for USY zeolites, the ion-exchange at 298 K with transition metal ions has been reported to be limited to 76% [39]). Comparison of XRD profiles (Figs. 1 and 2) of zeolite peaks before and after Ni loading (peaks at d values of 0.386, 0.329 and 0.297 nm) revealed a very small loss of crystallinity in the Ni/NaY samples (maximum 1.2%). The

Table 1 Some characterization data of calcined Ni/FAU and Ni/ZSM-5 catalysts Sample

Nia (wt.%)

NiOb (nm)

Pore volumec (cm3 g−1 )

BJH pore diameterc (nm)

Vd (cm3 (STP) g−1 )

BETc (m2 g−1 )

USY Ni(2.7)USY Ni(6.6)USY NaY Ni(3.3)NaY Ni(6.7)NaY ZSM-5 Ni(3.1)ZSM-5 Ni(6.0)ZSM-5

– 2.7 6.6 – 3.3 6.7 – 3.1 6.0

– – – – – * – – 6.6

0.24 0.21 0.19 0.29 0.27 0.24 0.12 0.11 0.11

3.8 5.9 3.9 2.6 4.5 4.9 3.5 5.0 4.5

186 130 147 204 184 204 99 29 66

662 551 513 657 650 569 331 326 345

a

As measured by plasma emission spectroscopy. Crystal size of the hexagonal NiO phase (ASTM 22–1189) as measured from X-ray diffraction; *: the NiO phase was detected but it was impossible to calculated the crystal size. c As measured by N adsorption–desorption isotherms. 2 d N adsorption at P/P 0 = 0.2. 2 b

B. Pawelec et al. / Applied Catalysis A: General 262 (2004) 155–166

N i/F A U

(f)

Linear counts (au)

(e )

(d )

*

*

*

(c )

(b )

(a )

20

40

60

Bragg's angle (º)

Fig. 1. XRD patterns of calcined Ni/NaY and Ni/USY samples: (a) blank NaY; (b) Ni(3.3)NaY; (c) Ni(6.7)NaY; (d) USY; (e) Ni(2.7)USY; and (f) Ni(6.6)USY catalysts.

Ni/USY counterparts did not undergo any crystallinity loss at all. For all the Ni/ZSM-5 samples, the loss of crystallinity was also very low (in the 0.3–1.2% range). The effect of Ni2+ ion-exchange on the zeolites was also derived from the framework vibration IR spectra (not reported here). The framework absorption region in the 400–1400 cm−1 range included absorption bands assigned to the vibrations of Al(Si)O4 tetrahedra at about 544, 790 and 1068 cm−1 , and to internal vibrations at about 461, 1120 and 1222 cm−1 . The bands observed at 544 and 1220 cm−1 arise from 5-ring chain and 5-ring block vibrations, respectively (structure-sensitive peaks) [40]. For all

159

catalysts, comparison of the positions and intensities of the absorption bands revealed a small decrease in the intensity of the bands corresponding to the vibrations between the tetrahedra. The effect of Ni-loading was estimated from the optical density ratio of the 544 and 461 cm−1 bands. Since for all catalysts, this ratio did not change significantly, it was concluded, in keeping with the XRD data, that the crystallinity of the zeolites was preserved. The X-ray powder diffraction data were used to identify the Ni2+ species formed on the support surface (Figs. 1 and 2). The X-ray diffraction profiles of the Ni/USY, Ni/NaY and Ni/ZSM-5 zeolites were typical of crystalline FAU and ZSM-5 materials. As expected, the aluminium silicate phase (PDF 44-2) was found for all zeolites. Contrary to Ni/USY and Ni(3.3)NaY, the Ni(6.7)NaY and Ni(6.0)ZSM-5 samples exhibited diffraction lines of NiO crystals at 2θ values of 43.4 and 62.9 (PDF number 44–1159). For the Ni(6.0)ZSM-5 sample, the average NiO crystallite size (calculated by the Debye–Scherrer equation from XRD line broadening of the most intense 2θ line of 43.4) was 6.0 nm. In the case of the Ni(6.7)NaY catalyst, no attempt was made to calculate the NiO crystal size since the diffraction lines of the NiO phase were overshadowed by the strong diffraction lines of the bare NaY zeolite (Fig. 1). 3.1.2. Electronic spectroscopy Diffuse reflectance spectroscopy was employed to investigate the symmetry and the valence of the supported nickel. The DRS-UV spectra of nickel-exchanged zeolites are plotted in Fig. 3 as a Kubelka–Munk–Schuster function. The DR spectra of Ni/USY samples differed from those of the Ni/NaY samples. The Ni/NaY samples displayed a major band at 275 nm and a minor one at 720 nm. The former can be attributed to Ni2+ cations in trigonal (D3 h ) coordination in the sodalite cages (SI sites) [41]. Since the DR spectra were recorded from samples exposed to air, the presence of the small band at 720 nm may have arisen from nickel ions in the near-tetrahedral coordination. This situa-

Ni/ZSM-5 275

10

62.9

(c)

log (SKM)

Linear counts (au)

43.4

(b)

417

1

720

N i/Z S M -5

0.1

N i/N aY

(a)

N i/U S Y 0.01

20 40 60 Bragg's angle (º)

Fig. 2. XRD patterns of blank ZSM-5 zeolite (a), Ni(2.3)ZSM-5 (b), and Ni(6.0)ZSM-5 (c) calcined catalysts.

200

400

600

800

1000

W avelength (nm )

Fig. 3. Diffuse reflectance electronic spectra (Schuster–Kubelka–Munk function) for the oxide Ni/USY, Ni/NaY and Ni/ZSM-5 series of catalysts.

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tion occurs when, in addition to the framework oxygen ligands of the aluminosilicate structure, the nickel ions are coordinated with a fourth oxygen originated from a residual water molecule [42]. By contrast, the USY-supported samples showed two bands at 417 and 720 nm. Using X-ray and EXAFS data of Dooryhee et al. [43], these bands were assigned by Lepetit and Che [41] to distorted tetrahedral nickel species located in the sodalite cavities (SI sites) and/or supercages (SII sites). For Ni/ZSM-5 catalysts, the UV-Vis DRS spectra of the all catalysts displayed a band at 275 nm, which may have been due to Ni2+ –O charge-transfer transitions. In consonance with the XRD data, the Ni(6.0)ZSM-5 catalyst showed a greater intensity of the band at 275 nm than the Ni(3.1)ZSM-5 homologue. 3.1.3. Textural properties The textural properties of all nickel catalysts and bare zeolites were evaluated from nitrogen adsorption–desorption isotherms. Since the shape of the N2 adsorption–desorption isotherms of the Ni-containing zeolites were similar to those of the Ni-free zeolites, only the former are compared in Fig. 4. As can be seen, the N2 isotherm of Ni/NaY zeolites are of Type I whereas those of the Ni/USY and Ni/ZSM-5 catalysts are of Type I/II. For the Ni/USY and Ni/ZSM-5, the formation of a monolayer at low relative pressure is the prevailing process whereas at high relative pressure a multilayer adsorption take place [44]. The Type I/II isotherms are indicative of the micro- and macroporous structure of materials. The appearance of macropores in pure USY zeolite was rather surprising, although mixing this zeolite with alumina—a common practice by the manufacturer—could account for the larger adsorbed volume and typical hysteresis loop observed in the high relative pressure region. The isotherms of Ni/USY and Ni/ZSM-5 catalysts have hysteresis loops that, according to the IUPAC classification, belong

3

-1

Adsorbed Volume (cm (STP) g )

250

200

(f)

204

(e)

184 147

150

(d) (c)

130

100 66

(b) 50

29

(a) 0 0.0

0.2 0.4 0.6 0.8 0 Relative Pressure (P/P )

to type H3 whereas the hysteresis loops of the isotherms of Ni/NaY samples belong to type H4. Both type of hysteresis are indicative of the solids consisting of aggregates or agglomerates of particles forming slit shaped pores [44]. The almost horizontal plateau of the Ni/NaY isotherm is characteristic of an ideal microporous structure (monolayer adsorption takes place) with pores of uniform size and/or shape. On the contrary, for Ni/USY and Ni/ZSM-5 zeolites the type H3 of hysteresis is indicative of pores of non-uniform size and/or shape [44]. The BET areas and micropore volume together with the amounts of nitrogen adsorbed at a P/P 0 = 0.2 are summarized in Table 1. For the sake of a valid comparison, the N2 adsorption–desorption data were normalized to unit weight of zeolite. As expected, in comparison with the ZSM-5 medium-pore zeolite, the faujastite-type zeolites have a larger pore structure and a two-fold larger specific BET area. The blank NaY and Ni(3.3)NaY samples showed the largest specific BET area (ca. 650 m2 g−1 ). Comparing the BET area the zeolites before and after Ni loading, the most pronounced drop in BET specific after Ni-incorporation was shown by the Ni(6.7)NaY and Ni(6.6)USY samples. This means that a fraction of the micropores of the NaY and USY zeolites becomes inaccessible to the N2 molecule after ion-exchange of H+ by Ni2+ . For the Ni/ZSM-5 and Ni(3.3)NaY samples, the almost constant BET area would probably be due to the slight increase in the external area, which is not counterbalanced by the simultaneous lowering in micropore volume [45]. In order to compare textural changes with respect to unloaded zeolite and at the same time obtain information about the internal location of metal species, the N2 adsorption capacities, e.g. at P/P 0 = 0.2, where the monolayer on the wall of the mesopores can be formed, were determined (Table 1). The N2 volume (P/P 0 = 0.2) decreased markedly upon Ni-incorporation with respect to the bare zeolites (Table 1). This suggests partial occupation of the cavities, and/or blockage of their access by Ni species, of the NaY, USY and ZSM-5 zeolites. The pore diameter and pore volume data of the unloaded and nickel-loaded zeolites, as determined from numerical analysis of the N2 desorption data by the Barret– Harkins–Jura (BHJ) method, are compiled in Table 1. The decrease in pore volume followed the order: Ni/NaY > Ni/USY ∼ = Ni/ZSM-5. Fig. 5 shows the pore size distribution. For all catalysts, the pore size distributions were relatively narrow. The pore size distributions for the Ni(6.6)USY, Ni(6.7)NaY and Ni(6.0)ZSM-5 samples lay at ca. 4.1, 3.8 and 3.8 nm, respectively. Unfortunately, the BJH method is imprecise in the region below 1 nm, where the supercages of the USY and NaY zeolites appear.

1.0

Fig. 4. N2 adsorption–desorption isotherms at 77 K for the Ni/FAU and Ni/ZSM-5 zeolites: (a) Ni(3.1)ZSM-5; (b) Ni(6.0)ZSM-5; (c) Ni(2.7)USY; (d) Ni(6.6)USY; (e) Ni(3.3)NaY; and (f) Ni(6.7)NaY.

3.1.4. Temperature-programmed reduction Because the ease of reduction of nickel ions in USY and NaY zeolites decreases in the order supercages > sodalite cages > hexagonal prisms [46], the coordination effect in

B. Pawelec et al. / Applied Catalysis A: General 262 (2004) 155–166 1.2

161

643

4.1

(a)

731 798

0.8

3.7

966

Ni(2.7)USY USY

0.0 1.2 (b)

3.8

0.8

(f) 798

Ni(6.7)NaY

(e )

639 895

2

dVdes / d(logD) Pore Volume (cm3 /g)

0.4

H Consumption (au)

Ni(6.6)USY

(d ) (c )

613

Ni(3.3)NaY

0.4 517

NaY

780 749

900

(b )

0.0 1.2

(a ) 3.8

(c)

400

600

800

1000 1200

0.8

T e m p e r a tu r e (K )

Ni(6.0)ZSM-5

0.4

Fig. 6. Temperature-programmed reduction profiles for the oxide Ni/FAU and Ni/ZSM-5 catalysts: (a) Ni(3.1)ZSM-5; (b) Ni(6.0)ZSM-5; (c) Ni(2.7)USY; (d) Ni(6.6)USY; (e) Ni(3.3)NaY; and (f) Ni(6.7)NaY.

Ni(3.1)ZSM-5 ZSM-5

0.0 2

4

6

Pore Diameter (nm) Fig. 5. The pore volume distribution of the Ni/USY (a), Ni/NaY (b) and Ni/ZSM-5 (c) catalysts as determined from N2 adsorption–desorption isotherms at 77 K.

zeolites is particularly important [47]. Indirect information about the localization of Ni-species in zeolites was obtained from TPR data. The observation of several peaks in the TPR profiles of the Ni/FAU samples (Fig. 6) indicated the presence of several different reduction sites. The peak at the lowest reduction temperature (maximum at 639 K) can be assigned to the reduction of NiO particles on the outer surface of the zeolite crystals, whereas peaks in the middle range of temperatures (maxima at 731 and 798 K) may be due to the reduction of Ni2+ (to Ni0 and Ni+ ) in supercages (SIII sites)

and sodalite cages (SII and SI sites faced at the hexagonal prisms), respectively, as observed for Ni/NaX zeolites [48]. Finally, the higher reduction temperature peak, with maximum at 966 K, might be associated with the reduction of nickel species located in hexagonal prisms. Comparing the TPR profiles of Ni/NaY and Ni/USY catalysts, it is more likely that the latter would have larger amount of the nickel species located in the sodalite cages than the former and a lower amount of NiO located on the outer zeolite surface. Noticeably, the reduction of nickel species in the Ni/ZSM-5 occurred at lower temperatures than in its Ni/NaY counterparts. The Ni/ZSM-5 sample showed peak maxima at 517, 613, 743 and 900 K. The two former peaks could be assigned to the reduction of NiO of different crystal size located on the outer zeolite surface, in accordance with the XRD data.

Table 2 Acidity of sulphideda Ni/FAU and Ni/ZSM-5 catalysts Sample

Acid sites concentrationb (mmol NH3 g−1 cat ) Weak

USY Ni(2.7)USY Ni(6.6)USY NaY Ni(3.3)NaY Ni(6.7)NaY ZSM-5 Ni(3.1)ZSM-5 Ni(6.0)ZSM-5 a b c

1.8 4.0 3.0 0.6 0.5 0.9 0.4 1.0 1.5

strengthb

400–550 Kc

Medium 7.5 3.8 7.2 0.6 1.1 1.7 2.3 3.9 1.5

strengthb

550–700 Kc

Strong

strengthb

0.6 2.4 1.3 0.1 0.0 0.0 0.5 1.0 0.8

The catalyst sulphidation was performed employing 10% H2 S/H2 mixture at 672 K for 2 h. Amount of desorbed ammonia for weak, medium and strong acidity. Temperature range for weak, medium and strong acidity at 1902 and 1941.

700–900 Kc

Total acid sites concentration (mmol NH3 g−1 cat ) 9.9 10.2 11.5 1.3 1.6 2.6 3.2 5.9 3.8

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3.2. Characterization of sulphided and used catalysts

3.2.2. FTIR spectroscopy of NO FTIR of adsorbed NO was used to obtain information about the sulphidability of nickel species on zeolites. Since the presence of large NiO crystals in the Ni(6.7)NaY and Ni(6.0)ZSM-5 catalysts (Table 1) might overshadow the support effect, the FTIR technique was employed only for low Ni-content samples. Fig. 7 compiles the IR spectra of adsorbed NO on catalysts after sulphidation at 673 K for 2 h. With the exception of the Ni(3.3)NaY sample, the Ni(2.7)USY and Ni(3.1)ZSM-5 catalysts showed two bands at ca. 1902 and 1838 cm−1 , associated with oxide (O) and sulphided (S) Ni2+ sites, respectively. The first band comes from the stretching vibration mode of a mononitrosyl adsorbed onto non-sulphided Ni2+ ions, whereas the second one at about 1838 cm−1 comes from the NO molecule adsorbed onto sulphided Ni2+ ions [49,50]. An estimate of the relative degree of sulphidation of the catalysts was obtained by comparing the intensity of the (S) and (O) peaks [Isulf /Ioxi ratio], once they had been normalized to nickel content. Considering the zeolite support effect, it can be seen that sulphidation degree follows the order NaY ∼ = ZSM-5 > USY. The degree of sulphidation determined by FTIR for low Ni-content samples agrees with the surface exposure of Ni species determined by the XPS technique (see below). Thus,

1839 1904

Absorbance (ua)

3.2.1. Temperature-programmed desorption (TPD) of ammonia The acid strength and the amount of acid sites after catalyst sulphidation at 673 K for 2 h were determined from ammonia temperature-programmed measurements (profiles are not shown here). The ammonia molecule is sufficiently small (cross-sectional area 0.141 nm2 ) to enter the pores of the ZSM-5 zeolite and to adsorb onto its Brønsted and Lewis acid sites. Since the TPD of ammonia represents a dynamic measurement of a thermodynamic property, the strength of acid sites is related to the corresponding desorption temperature. Table 2 compiles weak, medium and strong acidity data obtained after Gaussian deconvolution of the peaks. Considering the strength distribution depicted by the maximum temperature, the acid sites were arbitrarily classified into weak (400–550 K), medium (550–700 K), and strong strengths (700–900 K). Considering the concentration of total acid sites, expressed as mmol of ammonia desorbed per gram of catalyst, a strong effect of the support is observed. As a general trend, total acidity decreased in the order: Ni/USY > Ni/ZSM-5 > Ni/NaY samples. The medium acid site concentration increased in the Ni/NaY sample as compared with the bare NaY. For the Ni/ZSM-5 catalysts, the total acidity trend was Ni(3.1)ZSM-5 ∼ = Ni(6.0)ZSM-5 > ZSM-5. Considering the NH3 desorption temperature, the Ni(6.0)ZSM-5 sample exhibits a higher strong acidity than Ni(3.1)ZSM-5 catalyst (780 versus 749 K). This means that the former sample has stronger acid sites than the latter.

1836 (c ) (b )

1843 1900

(a )

1950

1900

1850

1800

W a v e n u m b e r (c m

1750 -1

)

Fig. 7. Net FTIR spectra of NO chemisorbed on sulphided (673 K, 2 h) Ni(3.1)ZSM-5 (a), Ni(3.3)NaY (b) and Ni(2.7)USY (c) catalysts.

the easier sulphidation of nickel phases on the NaY than on ZSM-5 and USY zeolites is in keeping with the larger surface exposure of Ni on the Ni(3.3)NaY than on both the Ni(3.1)ZSM-5 and Ni(2.7)USY samples. 3.2.3. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy was employed to investigate the surface composition and chemical state of the used catalysts. Table 3 compiles the binding energy of the Ni 2p3/2 photoelectron along with the surface Ni/Si atomic ratio. The S 2p spectral region of all the nickel catalysts showed a single peak at ca. 162.1 eV, which is characteristic of S2− ions. All samples showed a major component at ca. 853.4 eV, which corresponds closely to the value reported in literature for nickel sulphide (Ni3 S2 ) species [36]. Observation of the satellite line can be taken as being conclusive of the presence of Ni2+ ions in an environment of oxide ions at the surface. This finding, together with the second comTable 3 Binding energies (eV) and surface atomic ratios of used Ni/FAU and Ni/ZSM-5 catalysts by XPS Catalyst

Ni(2.7)USY Ni(6.6)USY Ni(3.3)NaY Ni(6.7)NaY Ni(3.1)ZSM-5 Ni(6.0)ZSM-5

Ni/Si atom × 103

Ni 2p3/2 a

XPS

XPS/bulkb

O

36 96 259 371 44 167

1.1 1.1 5.8 3.9 1.6 6.4

857.2 856.5 856.2 856.1 855.8 855.9

S (69) (64) (62) (68) (65) (77)

853.7 853.7 853.6 853.5 853.4 853.8

(31) (36) (38) (32) (35) (23)

a The percentage contributions of nickel sulphide species to total signal are given in parenthesis of BE data. b Ni/Si atomic ratio from XPS of used catalysts vs. the Ni/Si bulk atomic ratios obtained from chemical analysis.

B. Pawelec et al. / Applied Catalysis A: General 262 (2004) 155–166

ponent of Ni 2p3/2 peak at ca. 855.9 eV, which is ascribed to the non-sulphided Ni2+ ions interacting with zeolite lattice oxygen (Ni–OL ) [35], can be taken as conclusive that during sulphidation in a H2 /H2 S (10:1) mixture at 673 K for 2 h under atmospheric pressure the nickel is not completely sulphided. In the case of the USY-supported catalyst, the BE component of the Ni 2p3/2 peak at ca. 855.9 eV may also result from the Ni2+ ions interacting with the extra framework alumina (EFAL) produced in the zeolite pores during steaming [35]. Quantitative XPS data clearly show that the Ni/Si ratio depends on the type of zeolite used and it is much higher for the Ni/NaY than for the Ni/USY and Ni/ZSM-5 samples. As expected, the Ni/Si ratio in zeolites with low Ni-content was lower than in the high nickel content samples. The much larger surface (XPS) to bulk Ni/Si atomic ratio of the Ni(6.7)NaY compared to Ni(6.6)USY (3.9 versus 1.1, respectively) confirms a more homogeneous distribution of Ni species in the latter sample. However, the Ni/Si atomic ratios on the surface are higher than those in the corresponding bulk materials, indicating that the outer catalyst surface becomes Ni-enriched. The extent of sulphidation, based on the atomic concentration ratios of the nickel sulphide species, was similar in the 31–38% range for the Ni(6.6)USY, Ni(3.3)ZSM-5, Ni(3.3)NaY and Ni(6.7)NaY samples, and much lower for Ni(6.0)ZSM-5 (23%). In the case of the Ni(6.7)NaY sample, there is some discrepancy between the N2 adsorption–desorption and the XPS data. The decrease in the BET surface area as compared to NaY zeolite (from 651 to 569 m2 g−1 ) suggests that the Ni-species would be located in the pores of this zeolite. In addition, the high Ni/Si atomic ratio (Table 3) is indicative of the location of Ni species on the surface. Since the large NiO crystals are detected by XRD (Table 1), the decrease in specific BET area is more likely to be due to some blockage of the microporous openings by the large Ni-species located on the catalyst surface. For this sample, the almost constancy of the N2 volume adsorbed at P/P 0 = 0.2 is indicative of the free access of N2 into the mesopores of the Ni(6.7)NaY sample. 3.3. Simultaneous 1-pentene HYD and thiophene HDS The simultaneous hydroisomerisation of 1-pentene and HDS of thiophene on nickel containing USY, NaY and ZSM-5 catalysts was investigated at 10 bar hydrogen pressure and a temperature of 523 K. Our main objective was to lower olefin hydrogenation with respect to a commercial CoMo/Al2 O3 catalyst tested at the same conditions, while simultaneously increasing the HDS function of the catalysts. The intrinsic thiophene HDS and 1-pentene HYD activities of the Ni/FAU and Ni/ZSM-5 catalysts, expressed as mmoles of thiophene/1-pentene converted per hour and gram of catalyst, under steady-state conditions, are compared in Table 4. The CoMo/Al2 O3 reference catalyst is also included for comparison. The rate

163

of the HDS reaction at 523 K after 7 h follows the order: CoMo/Al2 O3 > Ni(6.7)NaY > Ni(3.1)ZSM-5 ≈ Ni(3.3)NaY > Ni(6.6)USY > Ni(6.0)ZSM-5 ∼ = ZSM-5 > Ni(2.7)USY. In comparison with the more active Ni(6.7)NaY sample, a commercial CoMo catalyst showed greater HDS intrinsic activity, but the difference was relatively low (0.19 versus 0.14 mmol h−1 g−1 ). The greater HDS activity of the CoMo sample is expected, considering the much larger metal content of phases that are active in the HDS reaction (2.7 wt.% of Co and 9.5 wt.% of Mo) and the ability to develop a “Co–Mo–S”phase, which exhibits a high intrinsic HDS reaction rate. The increase in HDS activity on passing from Ni(2.7)USY to Ni(6.6)USY is obviously due to the larger amount of Ni3 S2 phase formed on the latter (Table 3), in good agreement with previous studies [50]. For the ZSM-5 series, the increase in Ni-content from 3.1 to 6.0 wt.% led to a lower HDS reaction rate. This reduction in HDS activity could be related to the large NiO crystals detected by XRD diffraction on the Ni(6.0)ZSM-5. The intrinsic HYD activity trend is different from that observed in HDS: Ni(6.0)ZSM-5 ≈ CoMo/Al2 O3 > Ni(6.7)NaY > Ni(3.1)ZSM-5 > Ni(3.3)NaY > ZSM-5 > Ni(2.7)USY ∼ = Ni(6.6)USY. The higher HYD activity observed for the Ni(6.0)ZSM-5 sample decreased substantially for the Ni(6.7)NaY (ca. 34%) and Ni(3.1)ZSM-5 (ca. 48%) samples, and was strongly inhibited (86.2%) in the case of the Ni(6.6)USY sample. With the exception of Ni/USY samples, the increase in the Ni content in Ni/ZSM-5 and Ni/NaY zeolites leads to an increase in the hydrogenation capacity of the catalysts. Since catalyst deactivation is a problem common to the Ni/FAU and Ni/ZSM-5 systems (see last column in Table 4) and the catalysts showed unstability during first 2 h on-stream, the deactivation in HDS reaction was calculated from the ratio [(Xt=2 h − Xt=7 h )/(Xt=2 h )], where X is the thiophene conversion. For the catalyst deactivation, the trend observed in Table 4 is: Ni(2.7)USY > ZSM-5 > Ni(3.3)NaY > Ni(6.7)NaY ≈ Ni(6.6)USY > Ni(3.1)ZSM-5 > Ni(6.0)ZSM-5. In accordance with the acidity data (Table 2), Ni(2.7)USY catalyst showed a stronger degree of deactivation in the HDS reaction than the Ni/NaY and Ni/ZSM-5 counterparts with much lower acidity. The large acidity of the Ni(6.6)USY sample (Table 2) is responsible in part of the deactivation of this catalyst during first 2 h on-stream operation (data not presented here). Thus, the larger intrinsic activity of the Ni(6.7)NaY sample as compared to Ni(6.6)USY in the HDS reaction could be related to the lower deactivation (linked with lower acidity; Table 2) as well as to the greater exposure of the nickel surface on the former as compared to the latter (see XPS to bulk Ni/Si ratio, Table 3). Since thiophene HDS is very sensitive to the surface structure of the active sulphide phase [51], it is more likely that thiophene adsorption would be favored on the large Ni3 S2 phases formed on the outer surface of the Ni(6.7)NaY samples as compared to the small Ni3 S2 phases homogeneously distributed within the Ni(6.6)USY.

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Table 4 Specific reaction ratesa for thiophene HDS and 1-pentene HYD, and selectivities toward skeletal isomerisation of 1-pentene for Ni-containing zeolite catalystsa Catalyst

rHYD (mmol h−1 g−1 )

rHDS (mmol h−1 g−1 )

Skeletal ISO (%)

Deactiv.b (%) HDS

Ni(2.7)USY Ni(6.6)USY Ni(3.3)NaY Ni(6.7)NaY ZSM-5 Ni(3.1)ZSM-5 Ni(6.0)ZSM-5 CoMo/Al2 O3

1.85 1.31 4.69 6.20 3.60 5.08 9.55 9.49

0.02 0.09 0.09 0.14 0.03 0.10 0.08 0.19

1.1 2.7 0.9 0.4 9.2 20.5 15.4 5.3

72.0 38.8 50.4 42.5 58.2 25.3 10.6 0.0

Reaction conditions were: T = 523 K, P = 10 bar, WHSV = 10 h−1 , TOS = 7 h; CoMo/Al2 O3 catalyst is used as reference. Deactivation in HDS reaction calculated from the ratio [(Xt=2 h − Xt=7 h )/(Xt=2 h )], where X is the thiophene conversion.

The relatively high HDS activity of the blank ZSM-5 sample is in consonance with previous findings [29]. Considering the NH3 TPD data (Table 2) and the absence of the formation of tetrahydrothiophene (THT), the reaction may involve the cracking of the thiophene molecule on the acid sites of the ZSM-5 zeolite. Another explanation envisages the formation of SH– groups on the ZSM-5 zeolite [52]. Korányi et al. [53] proposed that SH– groups bonded to strong Brønsted acid sites would play a role in HDS catalysts working under conditions approaching industrial practice. This is because SH– groups have a dual function, i.e. as source of hydrogen and as Brønsted acid sites [54]. In contrast to ZSM-5, a very small amount of H2 S adsorbs dissociatively on the NaY zeolite [55].Thus, the strong HDS reactivity of both NaY-supported catalysts comes almost exclusively from the Ni species whereas the reactivity of the blank ZSM-5 could be linked to H2 S dissociation on Brønsted acid sites. On considering the balance of HDS versus HYD active sites under steady-state conditions (Fig. 8), measured by the ratio of the HDS and HYD reaction rates, it is clear that the best balance of the HDS and HYD functions was achieved over the Ni(6.6)USY sample, which showed an HDS/HYD ratio about three-fold larger than the commercial CoMo/Al2 O3 catalyst. On the basis of the catalyst

structure–activity relationship, it can be inferred that the catalytic response of Ni(6.6)USY catalyst must be related to a homogeneous distribution of nickel species within the zeolite pores and its outer surface (XPS to bulk ratio = 1.1, Table 3) and the greater acidity of the USY zeolite as well (Table 2). The influence of both factors can be deduced indirectly from Fig. 9, which shows the time-course of the HDS/HYD ratio (calculated from the ratio HDS conver-

6 N i/U S Y HDS-to-HYD ratio

b

4 N i(6 .6 )U S Y 2 N i(2 .7 )U S Y

0

2

4

6

8

10

6 N i/N a Y HDS-to-HYD ratio

a

4

N i(6 .7 )N a Y

N i(3 .3 )N a Y

2

8

0

6

4

6

8

10

N i(3 .1 )Z S M -5 N i(6 .0 )Z S M -5 Z S M -5

N i( 6 .7 ) N a Y 2

0

a

b

c

d

e

f

g

CoM o

HDS-to-HYD ratio

4

/r

HDS HYD

r

2

6

2

x 10 ratio

N i( 6 .6 ) U S Y

4

2

C a t a ly s t

Fig. 8. Comparison of the HDS/HYD ratio (calculated from rate of HDS reaction to rate of HYD reaction) for Ni/FAU and Ni/ZSM-5 catalysts: (a) Ni(2.7)USY; (b) Ni(6.6)USY; (c) Ni(3.3)NaY; (d) Ni(6.7)NaY; (e) ZSM-5; (f) Ni(3.1)ZSM-5; (g) Ni(6.0)ZSM-5. As reference, the activity data of a commercial CoMo/Al2 O3 (at 498 K) catalyst are included. Reaction conditions were: P = 10 bar; T = 523 K, WHSV = 10 h−1 , TOS = 7 h.

0

2

4 T O S (h )

6

8

10

Fig. 9. Time curse for HDS/HYD ratio (calculated as HDS conversion to HYD conversion) for Ni/USY (a) Ni/NaY (b) and Ni/ZSM-5 (c) catalysts. Reaction conditions were: P = 10 bar; T = 523 K, WHSV = 10 h−1 .

Thiophene HDS and 1-pentene HYD (%)

B. Pawelec et al. / Applied Catalysis A: General 262 (2004) 155–166 30 Ni(6.6)USY

20 HDS

10 HYD 0

0

2

4

6

8

Time on-stream (min)

Fig. 10. Time course for thiophene HDS and 1-pentene HYD over Ni(6.6)USY catalyst. Reaction conditions as in Fig. 9.

sion to 1-pentene conversion to paraffins) for the Ni/USY, Ni/NaY and Ni/ZSM-5 catalysts. Ni(6.6)USY was the only sample showing an increase in the HDS/HYD ratio with time on-stream. For this catalyst, the greatest deactivation in 1-pentene HYD with respect to thiophene HDS is observed in Fig. 10, which shows HDS and HYD conversions over the Ni(6.6)USY sample with the time on-stream. Thus, the increase in the HDS/HYD ratio with time on-stream is due to a stronger deactivation of the sites responsible for 1-pentene HYD. Recently, Hatanaka et al. [16] has reported evidence that thiophene and olefins show different types of response to H2 S addition, and that n-olefin hydrogenation is inhibited by H2 S. Based on the differences in response, different types of active sites for HDS and n-olefin reactions appear to be present [3]. Thus, for Ni(6.6)USY sample the strong deactivation of HYD sites can be tentatively ascribed to H2 S inhibition of 1-pentene HYD [3]. The lower deactivation of the HDS sites points to their higher resistance toward deactivation by the H2 S evolved from HDS reaction compared to HYD sites. 3.4. Isomerisation of 1-pentene In terms of 1-pentene isomerisation, the FAU-supported catalysts showed almost exclusively double bond isomerisation (data not presented here) whereas both double bond and skeletal isomerisation were observed in the ZSM-5 zeolite and its nickel-loaded counterparts. The skeletal isomerisation data are compiled in Table 4. According to the literature, the prerequisite necessary for high double-bond 1-pentene isomerisation is the low acid strength of the catalytic sites [38]. However, in this study no correlation between week-strength acid sites (Table 2) and selectivity toward double bond isomerisation of C5 = was observed. Since the acidity measurements were performed on fresh sulphided samples, a possible explanation for this phenomenon would be the modification of the acid sites during on-stream conditions by coke.

165

The necessary prerequisite for skeletal 1-pentene isomerisation is slightly different than for double isomerisation, since besides the medium acid strength of the catalytic sites [38], a suitable catalyst pore size is necessary [56]. The following trend can be established for skeletal isomerisation over sulphided nickel catalysts: Ni(3.1)ZSM-5 > Ni(6.0)ZSM-5 > ZSM-5 ∼ = Ni(6.6)USY > Ni(2.7)USY > Ni(3.3)NaY > CoMo/Al2 O3 . Thus, in agreement with previous observations [24], the ZSM-5-based samples only, and particularly that with the lowest Ni-loading, are suitable for skeletal isomerisation, but in no case the USY- or NaY-supported nickel ones. In spite of these unfavorable shape configurations, the Ni(6.6)USY sample showed a low skeletal isomerisation (2.7%). In sum, taking into account that minimization of olefin hydrogenation is more important than their isomerisation, the Ni(6.6)USY catalyst with most favorable HDS/HYD ratio might be considered as a promising candidate for the hydrotreatment of FCC naphtha. This catalyst is the only one which possesses the nickel species located within the zeolite structure. From UV-Vis diffuse reflectance spectra (Fig. 3) and TPR data (Fig. 6) it is inferred that nickel oxide species are located in the sodalite cavities (SI sites) and/or supercages (SII sites). These species appear to be rather well fixed in that locations since no changes were observed by XPS in the catalyst sample subjected to sulphidation (data not presented here) and/or on-stream conditions (Table 3).

4. Conclusions From the catalytic behaviour of the Ni/FAU and Ni/ZSM-5 systems in simultaneous thiophene and 1-pentene transformations, it can be concluded that the HDS of thiophene and HYD of 1-pentene reactions take place on different active sites. The Ni(6.6)USY catalyst appears to be the most appropriate candidate for achieving efficient HDS, at the same time minimizing the olefin hydrogenation reaction. On the basis of joint analysis of the catalyst characterization data and catalyst performance in the target HDS and HYD reactions, it can be inferred that the catalytic response is linked to a homogeneous distribution of nickel species within the zeolite structure and the stronger acidity of the USY zeolite as well. This study provides experimental evidence that the Ni/USY catalyst with a high Ni content can be used either alone or mixed with traditional FCC catalysts to favor deep HDS, while at the same time minimizing the olefin hydrogenation reactions.

Acknowledgements B.P and R.M.N. acknowledges financial support from the Spanish Ministry of Science and Technology (Ramón y Cajal Project).

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