SiO2-Al2O3 catalysts; support acidity effect

Applied Catalysis A: General 264 (2004) 43–51

Hydrogenation of aromatics over Au-Pd/SiO2 -Al2 O3 catalysts; support acidity effect夽 A.M. Venezia a,∗ , V. La Parola b , B. Pawelec b , J.L.G. Fierro b a

Istituto per lo Studio dei Materiali Nanostrutturati, ISMN-CNR Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy b Istituto de Catalysis y Petroleoquimica, CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain Received in revised form 16 December 2003; accepted 16 December 2003

Abstract Bimetallic Au-Pd catalysts supported on amorphous silica-alumina (ASA) with vary amount of alumina (0, 8, 14, 28 and 100%) were prepared by the simultaneous reduction of palladium and gold precursors by ethanol in the presence of the polyvinylpyrrolidone (PVP). X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses indicated formation of alloyed Au–Pd particles. Measurements of temperature-programmed desorption (TPD) of ammonia were performed to monitor the acid strength and the amount of acid sites on the catalysts after their reduction with 10% H2 /Ar at 573 K for 1 h. The effect of the support acidity on the catalytic activity was investigated in the simultaneous hydrogenation (HYD) of toluene (T) and naphthalene (NP) in the presence of dibenzothiophene (DBT). The amount of coke formed during the catalytic tests was also determined. Under the selected conditions (P = 5.0 MPa, T = 523 K, WHSV = 41.2 h−1 ), all the catalysts were resistant to poisoning with 113 ppm of S (as DBT). The hydrogenation activity of toluene and the HDS activity of DBT correlate with the concentration of the medium strength acid sites. The enhancement of the HYD activity and the S-tolerance were related to modifications of the electronic properties of the metal atom upon interaction with the acid sites and upon intermetallic interaction. © 2003 Elsevier B.V. All rights reserved. Keywords: Aromatics hydrogenation; Silica; Alumina; Au-Pd catalysts

1. Introduction More stringent requirements for reducing the content of aromatics and sulfur in fuels have brought increasing attention to new catalysts involved in hydrotreating process. For deep hydrogenation (HYD) of aromatics and deep hydrodesulfurization (HDS) in diesel fuel, the single- and two-stage approaches have been proposed. The single stage method combines severe hydrodesulfurization with aromatics hydrogenation (HDA) by the use of a conventional CoMo, NiMo, or NiW catalyst at high temperature and H2 pressures substantially higher than the H2 pressure at which current HDS units operate [1,2]. In the two-stages method, the conventional hydrotreating catalyst is used in 夽 Part of the content of this paper appeared in the Proceedings of the Gold 2003: New Industrial Applications for Gold; Conference held in Vancouver, Canada. ∗ Corresponding author. Tel.: +39-09-16809372; fax: +39-09-16809399. E-mail address: [email protected] (A.M. Venezia).

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.12.025

the first reactor and a noble-metal catalyst in the second reactor [3]. This method yields a low aromatic diesel stream at moderate hydrogen pressure. However, noble metals are very susceptible to sulfur poisoning [4]. Since the use of noble metal catalysts is limited by the severe pretreatment conditions in the first reactor, the commercial experience with sulfur tolerant noble metal catalysts is limited [1,5]. The high intrinsic hydrogenation activity and their sulfur tolerance may be enhanced, modifying the physicochemical characteristics of the metal atoms by: (i) changing the acid–base properties of the carrier [6]; (ii) changing the metal particle size or (iii) alloying [7]. Concerning the former, it was reported that the sulfur tolerance of Pt or Pd catalysts can be greatly enhanced by using acidic supports such as zeolites [1,5–11] whereas less acidic supports, such as SiO2 –Al2 O3, can generate moderate [5,12–14] or large sulfur resistance [7]. The high S-tolerance of zeolite-based Pt or Pd catalysts is commonly attributed to the formation of electron-deficient metal particles, Pt␦+ or Pd␦+ , upon interaction of the reduced metal with the Brønsted acid sites of the zeolite, which in turn lowers the strength of the S–Me

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bond [13]. The decrease of the particle size, by determining the electron-deficiency of metal particles, may also produce significant changes in the turnover frequencies of aromatic hydrocarbon hydrogenation on small Pt and Pd metal clusters in a zeolite framework [9,15–17]. In the case of large metal crystallites, with the electronic properties similar to those of bulk metal [18], an alternative explanation for the enhanced activities of Pd and Pt, supported on acidic carriers, involves a Langmuir–Hinshelwood model, which invokes dual-site adsorption, both on the metal and on the support [13,14,19]. Concerning the alloy formation, two effects can arise; a “geometric” effect, represented by the variation in composition, configuration and availability of active sites for a given reaction and the so-called “ligand-effect”, which evokes electronic interactions between electronically dissimilar components. Recent studies had demonstrated the improved sulfur tolerance of a palladium-gold catalyst attributed to the “ensemble” size effect of the alloyed Au–Pd particles, when supported on silica and amorphous silica-alumina [20,21]. The two metals, gold and palladium, present in the catalysts, prepared either by impregnation with colloidal dispersion of the two metal particles, either by the slow deposition precipitation method, exhibited large synergy effect in HDS of thiophene and dibenzothiophene [20,21]. The synergy was explained in terms of the strong affinity of gold with the sulfur-containing substrate and the strong activation of the hydrogen molecule by palladium. The use of Au in hydrogenation reactions has been investigated relatively recently [22–33]. This is because gold with a completely filled Au d-band has a limited ability to dissociate H2 molecules [22]. In line with this, our study on Pd and Au supported on two types of alumina materials in the simultaneous hydrogenation (HYD) of naphthalene (NP) and toluene (T) in the presence of dibenzothiophene (DBT) confirmed the larger HYD capability of the Pd compared to Au catalysts [23]. In spite of these differences in reactivity, the use of Au catalysts appears promising in HYD reactions when the factors controlling intramolecular selectivity are more important than large HYD activity [24]. Bailie and Hutchings [25,26] observed that pre-treatment of Au catalysts with thiophene increased the selectivity of but-2-en-1-ol in the gas-phase hydrogenation of but-2-enal at 393 K. The authors claimed that sulfur acts as a promoter for this selective hydrogenation reaction, rather than a poison as would have been expected from the commonly accepted poisoning effects of sulfur compounds on hydrogenation catalysts. Silica-, titania- and alumina-supported Au particles of 1–6 nm size, prepared by various synthetic routes were studied by Claus group in selective hydrogenation of acrolein, crotoaldehyde and 1,3-butadiene [24]. The very previous study on the reaction of cyclohexane with hydrogen on gold films [27] and gold powder [28] demonstrated that simultaneous dehydrogenation and hydrogenation of cyclohexane might occur. To our knowledge, the literature reports on the use of bimetallic Au-Pd catalyst for hydrogenation reaction is

rather scarce. The hydrogenation of 1,3-butadiene and isoprene on Au-Pd catalysts was investigated by Joice et al. [29], Bond and Rowle [30], Bond and Thomson [31], respectively. Carbon-supported palladium-gold catalysts were studied in the reaction of dichlorodifluoro ethane with dihydrogen [32]. It was found that the selectivity toward difluoromethane (desired reaction product) was increased upon introducing gold to palladium, and this enhancement depended very much on the degree of Pd–Au alloying. Acetylene hydrogenation and formation of surface deposits have been investigated on two series of Pd and Pd–Au/SiO2 catalysts differing in metal particle size [33]. The decoration of Pd by Au and the morphology of particles improved the ethene selectivity. On these premises, the aim of the present work was to ascertain the catalytic activity of Pd-Au catalysts in the hydrogenation of aromatics. By using aluminosilicate with varying alumina content [0,8,14,28, 100%], the influence of the support acidity on the structure of the supported Au-Pd catalysts and on their catalytic response, in the aromatics hydrogenation in the presence of S-compound (DBT), was evaluated. The hydrogenation of toluene and naphthalene mixtures was chosen as a model reaction because both substrates resemble the structure of aromatic compounds found in the diesel fraction (typical light gas oil (LGO)). Dibenzothiophene (DBT) was selected as the model compound for sulfur poisoning because it is one of the main S-containing compounds in diesel. Moreover, our previous study confirmed that Au–Pd alloy is active in the HDS of DBT [20]. Careful investigation of the catalyst structure was afforded by X-ray diffraction (XRD), temperature-programmed desorption (TPD) of NH3 and X-ray photoelectron spectroscopy (XPS) measurements in an attempt to establish a relationship between activity and catalyst structure.

2. Experimental 2.1. Catalyst preparation The colloidal dispersions of the Au/Pd clusters protected by the polymer poly(N-vinyl-2-pyrrolidone) (PVP) were obtained by an alcohol-reduction method [34]. The appropriate quantities of AuCl3 and PdCl2 were dissolved in 400 ml of an ethanol water (1/1 (v/v)) solution containing the PVP (MW = 10,000). The weight ratio of the PVP over the metal precursor was about 5. After adding the support to the solution, the suspension was stirred and refluxed at 363 K for 5 h under nitrogen. Darkening of the solution indicated reduction of the metal ions. The excess of liquid was removed in a rotary evaporator and the solid was washed several times to eliminate the free PVP and the chloride ions. Finally, the samples were dried in an oven at 343 K and then calcined in air at 673 K for 1 h. At this temperature, PVP decomposes completely. All

A.M. Venezia et al. / Applied Catalysis A: General 264 (2004) 43–51

reagents were from Aldrich. The following materials were employed as supports: SA-0, commercial sodium-free silica (Aldrich); SA-8 an amorphous silica-alumina (ASA) (Grace Davison Chemical; 8.8% of alumina); SA-14, an amorphous silica-alumina (Aldrich; 14% of alumina); SA-28, an amorphous silica-alumina (Grace Davison Chemical; SMR 5–473; 28% of alumina); SA-100, pure ␥-alumina (Aldrich).

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 dissolved in a mixture of HF, HCl and HNO3 at 363 K and homogenized in a microwave oven and then aliquots of solution were diluted to 50 ml using Mili-Q deionized water. 2.2.2. X-ray diffraction (XRD) The X-ray diffraction measurements for the structure determination were carried out with a Philips vertical goniometer using Ni-filtered Cu K␣ radiation. A proportional counter and a 0.05◦ step size in 2␪, from 2θ = 30 to 2θ = 50◦ , were used. The assignment of the various crystalline phases was based on the JPDS powder diffraction file cards [35]. The obtained XRD profiles were fitted using the software provided with the instrument. From the lattice parameter shifts, calculated from the angular position of the (1 1 1) and (2 0 0) metal reflections, according to Vegard’s law, the molar compositions x and y of the solid solutions Aux Pdy were obtained [36]. The precision on these calculated values was determined mainly by the error on the lattice parameters and was estimated of the order of 5%. The particle sizes of different phases were calculated from the line broadening of the most intense reflections using the Scherrer equation [37]. Estimated errors on particle sizes and on the relative percentages of the different phases are of the order of 10%. 2.2.3. Temperature-programmed desorption (TPD) of ammonia Ammonia-TPD measurements of the samples pre-reduced with 10% H2 /Ar at 573 K for 1 h were obtained on a semiautomatic Micromeritics TPD/TPR 2900 apparatus interfaced with a computer. Because ammonia can be adsorbed by hydrogen bond or dipolar interactions, ammonia was adsorbed at high temperature (at 473 K for 1 h) in order to overcome NH3 physisorption. TPD experiments were performed in a stream of He (Air Liquide) flow (50 ml/min) at a heating rate of 10 K/min up to 1073 K. The concentration of NH3 in the exit gas was continuously monitored by the TCD. The water evolved was trapped in a NaOH trap located just before the TCD. Because of peak overlapping, the semiquantitative comparison of catalyst acidity was accomplished using Gaussian deconvolution.

45

2.2.4. X-ray photoelectron spectroscopy The X-ray photoelectron spectroscopy analyses were performed with a VG Microtech ESCA 3000 Multilab, equipped with a dual Mg/Al anode. The spectra were excited by the non-monochromatised Al K␣ source (1486.6 eV) operated at 14 kV and 15 mA. The analyser operated in the constant analyser energy (CAE) mode. For the individual peak energy regions a pass energy of 20 eV across the hemispheres was used. Survey spectra were measured at 50 eV pass energy. The sample powders were pelletised and then mounted on a double-sided adhesive tape. The pressure in the analysis chamber was in the range of 10−8 Torr during data collection. The constant charging of the samples was corrected by referencing all the energies to the C 1s peak at 285.1 eV arising from adventitious carbon. The invariance of the peak shapes and widths at the beginning and at the end of the analyses indicated the absence of differential charging. The peaks were fitted by a non-linear least square fitting program using a properly weighted sum of Lorentzian and Gaussian component curves after background subtraction according to Shirley [38] and Sherwood [39]. For the exact determination of the Pd 3d5/2 and Au 4f5/2 binding energies, the overlapping Au 4d5/2 and Pd 4s peaks were included in the fitting procedure. The binding energy values are quoted with a precision of ±0.15 eV. Surface atomic concentration was evaluated from peak areas using appropriate sensitivity factors built in the VG instrument software. The samples used in the catalytic tests were kept in isooctane to avoid contact with air [6]. Thermogravimetrics measurements were performed with TGA/SDTA851e equipment (Mettler Toledo) employing 20% O2 /N2 mixture at heating rate 10 K/min from room temperature up to 1073 K. 2.3. Activity measurements Hydrogenation of aromatic compounds was performed in a continuous-down-flow catalytic reactor. The model feed consisted of DBT (113 ppm of S), naphthalene, and toluene dissolved in hexadecane. The molar flow rate of the toluene, naphthalene and DBT were 23, 1.8 and 0.04 mmol h−1 , respectively. Activity tests were performed using 0.25 g of catalyst diluted with SiC. The procedure for catalyst activation involved heating to a reduction temperature of 573 K (heating rate 0.4 K/min) in an H2 /N2 mixture (ratio 1:10 (v/v); flow rate 55 ml/min) at atmospheric pressure, followed by isothermal reduction at this temperature overnight. Catalytic activities were measured at steady state conditions (after 4 h of time on-stream), at T = 523 K, 5.0 MPa of total pressure, weight hourly space velocity (WHSV) of 41.2 h−1 and a H2 /feed ratio of 202 l(N)/l. Liquid samples were analyzed by GC with FID (Varian Model Star 3400 CX chromatograph) equipped with a 30 m×0.53 mm DB-1 column (J&W Scientific). Besides the unreacted model feed compounds (DBT, naphthalene and toluene), biphenyl (BP), cyclohexylbenzene (CHB), tetralin, decalin and methylcyclohexane were the

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only products detected. Activities are described in terms of the specific reaction rates calculated according to the Eq. (1): Xi F ri = (1) mcat

30

45

50

SA100 SA28 SA14 SA8 SA0 30

35

40

45

50

Fig. 1. X-ray diffractograms of the fresh Au-Pd/SA-x samples (after calcinations at 673 K).

3. Results and discussion Chemical composition and textural properties of the calcined catalysts are summarized in Table 1. Since all catalysts possess very low metal contents, their N2 adsorption–desorption isotherms (not shown here) did not vary with respect to the bare carriers. Therefore, in Table 1 only data of the bare supports are reported. Typically, according to the IUPAC classification, the N2 isotherms (not shown here) of the carriers is of type IV, whereas their hysteresis loop belong to type H2 [41]. The hysteresis loop proved to be relatively large and appeared at low relative pressures, indicating the presence of mesopores. The X-ray diffraction patterns of the fresh catalysts (calcined) are shown in Fig. 1. Reflection peaks typical of PdO and Aux Pdy alloy phases are present in the all sample patterns. The alumina catalyst exhibits also the broad bands of the ␥ alumina. Through the fitting of the main reflection peaks, the evaluation of the lattice parameters allowed to determine the metal atomic composition of the Au–Pd solid solutions with the corresponding particle sizes. The results of the XRD analyses on the fresh samples are summarized in Table 2, in which the various crystal phases along with the corresponding particle sizes and molar percentage are listed. As observed before, although the samples on an atomic concentration basis are richer in palladium, the obtained alloy phases are generally richer in gold [20,21]. The

Table 1 Chemical compositiona of oxidic Au-Pd/SA-x (x = 0, 8, 14, 28, 100 wt.% of Al2 O3 ) catalysts and textural properties of bare supportsb Catalyst

Au (wt.%)

Pd (wt.%)

BETb (m2 g−1 )

Vp b (cm3 g−1 )

dp b (nm)

SA-0 SA-8 SA-14 SA-28 SA-100

0.7 0.7 0.5 0.6 0.7

0.6 0.7 0.6 0.6 0.7

546 438 436 394 237

0.92 0.73 0.62 0.74 0.80

5.0 5.3 5.0 7.5 13.5

As determined by chemical analysis. BET surface area, total pore volume (Vp ) and average pore diameter (dp ) of the blank supports as measured from N2 adsorption. b

40

Pd Au Pd Au x y PdO

where ri is the pseudo-first order specific rate (mol/gcat h), Xi the conversion of reactant i (DBT, toluene or naphthalene), F the molar flow rate of the reactant i (mol s−1 ) and mcat is the catalyst weight (g). Turnover frequencies, TOF (s−1 ), were calculated from the reaction rate (mmole of reactant i converted per s) per exposed mmole of metal, computed from particle sizes of palladium oxide and Au–Pd alloy particles as determined by X-ray diffraction measurements [40].

a

35

alloy phases are stable under the reducing conditions of sample pre-treatment and test reactions. As previously shown for silica and ASA supported Au-Pd catalysts, the only noticeable change upon reduction is the formation of some “free” Pd from PdO [21,40]. It is worth noticing that the alloy particles are smaller on pure silica and larger on pure alumina. With the exception of the SA-8, an increase of the size with the increase of alumina content is observed. The ammonia temperature-programmed desorption measurements were performed to monitor the acid strength and the amount of acid sites on the catalysts after their reduction with 10% H2 /Ar at 573 K for 1 h. Ammonia molecule is sufficiently small (cross-sectional area 0.141 nm2 ) to enter the pores of the catalysts and to react with Brønsted and Lewis acid sites. TPD of ammonia represents a dynamic measurement of a thermodynamic property and the strength of acid sites is related to the corresponding desorption temperature. The TPD profiles with the fitted peaks are shown in Fig. 2. The peak temperature maxima together with the acid sites concentration obtained from the Gaussian deconvolution are reported in Table 3. Considering the strength distribution depicted by the maximum temperature, the acid sites were classified into the weak (400–550 K) and medium (550–700 K). The highest temperature component of the TCD signal of SA-0, SA-8, SA-14 and SA-28 samples appears to be related to some dehydroxylation of the silica and silica-alumina supports. From Table 3, the acid site distribution of the silica-alumina catalysts, SA-8, SA-14 and Table 2 Crystal phase of the fresh bimetallic Au-Pd/SA-x catalysts Catalysts

Crystal phases (size in nm) %

SA-0 SA-8 SA-14 SA-28 SA-100

PdO PdO PdO PdO PdO

(10.4) 18 (8.7) 42 (7.2) 61 (10.0) 59 (13.7) 29

Au47 Pd53 Au74 Pd24 Au65 Pd35 Au69 Pd31 Au76 Pd24

(3.9) 82 (21.7) 58 (12.3) 39 (14.5) 41 (22.4) 71

A.M. Venezia et al. / Applied Catalysis A: General 264 (2004) 43–51

TPD signal intensity (a.u.)

SA0

SA8

SA14

SA28

SA100

500

600 700 800 Temperature (K)

900

Fig. 2. Profiles of TPD of NH3 of the reduced Au-Pd/SA-x catalysts (He; rate 10 K/min).

SA-28 is similar, but relatively large differences in the acid strength are observed. Thus, on the basis of the temperature maxima in Fig. 2 and Table 3, the observed catalyst acidity trend is: SA-0 > SA-8 > SA-14 > SA-28 = SA-100. Table 3 Acidity of reduced Au-Pd/SA-x (x = 0, 8, 14, 28, 100 wt.% of Al2 O3 ) catalysts Sample

SA-0 SA-8 SA-14 SA-28 SA-100

Acid sites concentration (mmol NH3 g−1 cat ) Weak strength (400–550 K)

Medium strength (550–700 K)

Total acid sites concentration (mmol NH3 g−1 cat )

– 0.84 1.02 0.82 0.36

0.72 1.40 1.28 1.26 0.87

0.72 2.24 2.30 2.08 1.23

(544) (520) (518) (517)

(639) (614) (593) (571) (571)

The temperature maxima of the weak and medium acid sites are given in parenthesis.

47

In the other words, the increase in alumina content leads to a decrease in the strength of the acid sites. Considering total acid sites concentration (see last column in Table 3), the catalyst acidity decreased in the order SA-14 > SA-8 > SA-28 > SA-100 > SA-0. The chemical state of the elements and their relative proportions has been determined by photoelectron spectroscopy. The binding energies of Si 2p, Al2p, Pd 3d, Au 4f and C 1s, as reference line, were recorded. The corresponding values for the fresh catalysts are listed in Table 4 along with the XPS-derived atomic ratios. As previously found for silica and ASA supported Au-Pd catalysts, the Pd 3d spectra, characterised by the two spin-orbit components, Pd 3d5/2 and Pd 3d3/2 , 5.3 eV apart, contain two chemical components, one at low binding energy, due to metallic palladium and the other, at higher binding energy, due to palladium oxide [20,21]. The Au 4f7/2 binding energy found at ∼84 eV is typical of metallic gold. The lower value (83.4 eV), obtained for the sample SA-0, is indicative of Au–Pd alloy formation [21,42]. It is worth noticing that such shift is observed only in this sample characterised by the presence of an alloy more palladium enriched, present in higher percentage. Concerning the XPS quantitative analyses, metal concentration close to the corresponding bulk values, as determined by ICP-AES, are generally obtained. The only exception is represented by the catalyst on pure alumina for which the surprisingly low surface metal concentration is indicative of a rather poor metal dispersion as also confirmed by the XRD analysis. To investigate the effect of sulfur present in the feed on the activity of the supported Au-Pd catalysts, the simultaneous hydrogenation of toluene and naphthalene were conducted in the presence of DBT (S = 113 ppm) at 523 K with WHSV of 41.2 h−1 . The specific rates, expressed in mol/(gcat h), as a function of the alumina content in support are presented in Fig. 3. For all catalytic systems, the order of aromatic reactivity follows the sequence: naphthalene toluene > DBT. Thus, in line with previous findings [7,14], the hydrogenation of toluene is clearly more difficult than the hydrogenation of naphthalene. This could be related to

Table 4 XPS Au 4f7/2 and Pd 3d5/2 binding energies (eV) and surface atomic ratios of the fresh catalysts (after calcination at 673 K) Au-Pd/SA-x Sample

Au 4f7/2

Pd 3d5/2

Au/Si

Pd/Si

Si 2p

Al 2p

SA-0

83.4 (2.2)

334.6 (2.2) 58% 336.6 (2.2) 42%

0.03

0.06

103.5 (2.6)

–

SA-8

84.4 (2.0)

335.3 (2.4) 38% 337.1 (2.4) 62%

0.03

0.08

103.8 (2.8)

75.4 (3.2)

SA-14

84.1 (2.0)

334.5 (1.8) 25% 336.6 (2.2) 75%

0.01

0.02

103.5 (2.7)

75.7 (2.8)

SA-28

84.3 (2.2)

335.2 (2.0) 18% 337.2 (2.2) 82%

0.06

0.1

103.6 (2.8)

75.4 (3.1)

SA-100

84.0 (1.6)

335.3 (1.7) 40% 337.0 (1.7) 60%

Au/Al 0.001

Pd/Al 0.002

–

74.9 (2.8)

The FWHM are given in parentheses. The relative atomic percentages of the Pd components are also reported.

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A.M. Venezia et al. / Applied Catalysis A: General 264 (2004) 43–51 330

Naphthalene

334

336

8 6 4

338

340

342

346

SA100

Toluene

1.0

SA28

0.5

DBT

SA-100

SA-28

SA-14

SA-8

SA-0

I (a.u.)

0.0

344

5.2 eV

3

-1

332

Experimental fit background Pd 3d Au 4d5/2

-1

r x 10 at 523 K (mol h . gcat )

10

SA14

Catalyst

Fig. 3. Pseudo-first order specific reaction rate for naphthalene (square), dibenzothiophene (top triangle), and toluene (circle) hydrogenation on the Au-Pd/SA-x catalysts as a function of the support alumina content. Reaction conditions were: T = 523 K, P = 5 MPa, and WHSV = 41.2 h−1 .

SA 8

SA 0 330

332

334

336

338

340

342

344

346

BE (eV)

Fig. 4. XPS Pd 3d of the spent catalysts.

fresh ones, are observed. The palladium 3d spectra exhibit only the metallic component with a slightly larger width as compared to the corresponding component in the spectra of the “fresh” sample. Most important, no S 2p component, appearing at 162 eV as metal sulfides, was detected in any of the used catalysts. XPS and X-ray diffraction analyses of similar catalysts supported on silica had clearly shown that, contrary to the monometallic Pd catalyst, no sulfide metal phases had formed in the presence of gold [20,21]. With the exception of the SA-8 catalyst, a general decrease of the

80

82

84

86

88

90

92

3.5 eV

Experimental fit background Au 4f

SA100

SA28 I (a.u.)

a decrease in the resonance energy per aromatic ring as well as to differences in the ␲-electron cloud density in the aromatic ring as result of the inductive effect of the methyl group [43]. For HYD of toluene, the order of intrinsic activity of the catalysts follows the sequence: SA-14 > SA-28 > SA-8 SA-0 > SA-100. Thus, with the silica-alumina carrier the HYD of toluene is clearly enhanced as compared with the silica and alumina counterparts. For the hydrogenation of the other substrates, the effect of the support is less pronounced. Considering HDS of DBT, our previous study demonstrated the following trend for the production of CHB at 593 K: Pd50–Au50/SiO2 Pd/SiO2 > Au/SiO2 [20]. Since the yield of CHB could be considered as indicative of the HYD capacities of the catalysts, a larger HYD capability of the bimetallic catalyst with respect to the Pd/SiO2 catalyst and Au/SiO2 sample is evident. As already observed in our previous works on SiO2 -supported catalysts [20,21], XRD measurements confirmed the formation of Pd–Au alloys for all binary catalysts. For all catalysts studied, tetralin was the major product in HYD of naphthalene, and yield of decalin was very small. As can be expected for aromatics hydrogenation of hydrocarbons containing more than one ring, the HYD of naphthalene over silica, silica-alumina and alumina catalysts clearly proceeds via successive steps (naphthalene → tetralin → decalin), [8]. The tetralin is formed first, relatively easy, and this process can be considered as an irreversible process because at T = 523 K and at hydrogen high pressure (P = 5 MPa), the equilibrium concentration of naphthalene was negligible [8]. All the catalysts were analysed by XPS after being used in the reactions. The Au 4f and Pd 3d spectra of the spent samples are shown in Figs. 4 and 5. The amount of coke, formed after a defined elapsed time on stream, was determined by thermogravimetry measurements for all the catalysts. The XPS and the TG results are listed in Table 5. As shown from the table, no significant changes of the Au 4f7/2 binding energies of the spent catalysts, with respect to the

SA14

SA8

SA0 80

82

84

86

88

90

BE (eV)

Fig. 5. XPS Au 4f of the spent catalysts.

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49

Table 5 Coke formationa , binding energies (eV) of core electrons and surface atomic ratios of usedb Au-Pd/SA-x catalysts Sample

Coke(%)

Au 4f7/2

Pd 3d5/2

Au/Si

Pd/Si

Si 2p

Al 2p

SA-0 SA-8 SA-14 SA-28 SA-100

11.7 9.0 10.2 13.6 11.1

84.1 83.9 83.8 84.6 84.4

335.8 335.5 335.5 336.2 335.8

0.01 0.03 0.002 0.03 Au/Al 0.001

0.04 0.08 0.01 0.06 Pd/Al 0.002

103.5 103.8 (2.8) 103.4 (2.4) 103.6 (2.8) –

– 74.3 75.1 75.9 74.9

(2.2) (2.4) (2.0) (2.8) (2.1)

(2.6) (2.0) (2.2) (2.7) (2.4)

(3.0) (2.6) (3.0) (2.6)

a

As determined by thermogravimetry. Tested in the simultaneous hydrogenation of toluene and naphthalene in the presence of DBT. Reaction conditions: T = 528 K; P = 5 MPa; WHSV = 41.2 h−1 . b

Au/Si and Pd/Si atomic ratios is observed in the catalysts after the reaction. Such decrease may be due to the preferential formation of coke on the metal particles. The unchanged atomic ratios of the SA-8 sample are therefore in accord with the lowest coke percentage formation, reported below. As mentioned in the introduction, we started to investigate Au-Pd catalysts in the aromatics hydrogenation after confirmation that Au–Pd alloy was active in HDS reaction [20,21]. Thus, in this work our objective was to investigate the influence of support composition on the Au–Pd alloy formation and to explore the potential use of the Au-Pd catalysts in aromatic hydrogenation in the presence of S-compound (DBT). As shown in Fig. 6, TOF values decreased in the order: naphthalene (NP) > toluene (T) > DBT. For HYD of naphthalene, SA-8 catalyst with large Au–Pd alloy particles (21.7 nm) showed the highest TOF value. This is probably due to easier flatwise adsorption of the NP [44] on the large Au–Pd alloy. In addition, a correlation between the concentration of medium strength acid sites (Table 3) and TOF in HYD of naphthalene and toluene was found. This is illustrated in Fig. 7. Noticeably, similar correlation for HDS of DBT was not found. The HYD activity enhancement in the presence of an acidic support has been noticed already in the hydrogenation of toluene [45]. This is because, in addition to metal centers, the acid sites of support might also play some role in aromatics hydrogenation as a consequence of H-spill over effects [18,19]. However, no correlation between the cata-

lyst acidity and the amount of coke was found. From the TG data compiled in Table 5, the amount of formed coke follows the trend: SA-28 SA-0 > SA-100 > SA-14 > SA-8. Surprisingly, the less acidic silica- and alumina-supported catalysts show higher coke formation than the more acidic SA-14 and SA-8. The large coke formation on SA-28 sample is in line with its largest total acidity. Multipoint centers for coke adsorption have been suggested as the sites which are first poisoned by coke [46]. It is likely that coke formation on SA-28 sample takes place on the acid sites located in the close vicinity of metal particles. Interestingly, the SA-8 sample with the largest TOF in hydrogenation of naphthalene showed the lowest coke formation in spite of its large acidity (see Table 3). This might be due to the fact that H-spillover not only enhances reaction rate but also decreases coke formation [47]. Independently from the support acidity, the S-tolerance of the Au-Pd catalysts was confirmed by the lack of any S peak in the X-ray photoelectron spectra [21]. The enhancement of the sulfur tolerance, in the case of noble metals on acidic carriers has been related to modifications of the electronic properties of the metal atom resulting from interaction with the Brønsted acid sites of the support (electron acceptor) [48–50]. The extent of such effect would be larger on smaller particles. In the present study, the sulfur tolerance

100 Naphthalene

-1 3

-1

TOF x 10 (s )

80

80 TOF x 10 (s )

Toluene NP DBT

3

60

40

60 40 Toluene

20 20

DBT

0 0

SA-0

SA-8

SA-14

SA-28

SA-100

0.8

1.0

1.2

1.4 -1

Medium strenght acidity (mmol NH3 gcat )

Catalyst

Fig. 6. TOF calculated for HYD of toluene and naphthalene, and HDS of DBT. Reaction conditions are as in Fig. 3.

Fig. 7. Correlation between the concentration of the medium strength acid sites (data from Table 3) and the TOF in HYD of toluene and naphthalene and HDS of DBT (data from Fig. 6). Reaction conditions are as in Fig. 3.

50

A.M. Venezia et al. / Applied Catalysis A: General 264 (2004) 43–51

exhibited by all the catalysts, characterized by large metal particles, has to be attributed mainly to the bimetallic interaction between gold and palladium [21,51,52]. Both effects, support acidity and alloy formation, would indeed lead to the formation of electron-deficient metal sites, which in turn lowers the strength of the S–M bond [48–50].

4. Conclusions The hydrogenation of toluene and naphthalene in the presence of sulfur was investigated in a silica-alumina supported Au-Pd bimetallic catalysts with varying alumina content (0, 8, 14, 28, and 100%) under conditions which approach those used in industry, namely an overall pressure of 5.0 MPa, reaction temperature of 523 K, and a WHSV of 41.2 h−1 . Upon activation in hydrogen at 573 K, the bimetallic Au-Pd/SA-14 showed a higher TOF in hydrogenation of toluene in the presence of sulfur (113 ppm). Formation of alloys in all samples was ascertained by XRD and by XPS analyses. The catalytic performance, in both, the hydrogenation of naphthalene and toluene and in the HDS of dibenzothiophene, depended on the acidity of the catalysts. In particular, a correlation between the concentration of the medium strength acid sites and the turn over frequencies in HYD of naphtalene and toluene was found. Coke formation was independent from the support acidity. The lack of any sulfur compound on the spent catalysts confirmed the positive effect of gold against sulfur poisoning.

Acknowledgements B.P acknowledges financial support from the Spanish Ministry of Science and Technology (Ramón and Cajal Project). VLP acknowledges University of Palermo for the “Young Reserchear Project”. Supports by the European Community, Grant COST project D 15 and by the cooperation program between CNR and CSIC are also acknowledged.

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