Isomerization of n-hexane over mono- and bimetallic Pd–Pt catalysts

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Applied Catalysis A: General 290 (2005) 97–109

Isomerization of n-hexane over mono- and bimetallic Pd–Pt catalysts

supported on ZrO2–Al2O3–WOx prepared by sol–gel

A. Barrera a, J.A. Montoya a,*, M. Viniegra b, J. Navarrete a, G. Espinosa a,A. Vargas a, P. del Angel a, G. Perez b

a Instituto Mexicano del Petroleo, Ingenierıa Molecular, Eje Central Lazaro Cardenas 152, C.P. 07730, Mexico, D.F., Mexicob Division de CBI, Universidad Autonoma Metropolitana-Iztapalapa, 09340 Mexico, D.F., Mexico

Received 23 November 2004; received in revised form 16 May 2005; accepted 18 May 2005

Available online 27 June 2005

Abstract

The catalytic performance of mono- and bimetallic Pd (0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts supported on ZrO2 (70, 85 wt.%)–Al2O3

(15, 0 wt.%)–WOx (15 wt.%) prepared by sol–gel was studied in the hydroisomerization of n-hexane. The catalysts were characterized by N2

physisorption, XRD, TPR, XPS, Raman, NMR, and FT-IR of adsorbed pyridine. The preparation of ZrW and ZrAlW mixed oxides by sol–gel

favored the high dispersion of WOx and the stabilization of zirconia in the tetragonal phase. The Al incorporation avoided the formation of

monoclinic-WO3 bulk phase. The catalysts increased their SBET for about 15% promoted by Al2O3 addition. Various oxidation states of WOx

species coexist on the surface of the catalysts after calcination. The structure of the highly dispersed surface WOx species is constituted mainly

of isolated monotungstate and two-dimensional mono-oxotungstate species in tetrahedral coordination. The activity of Pd/ZrW catalysts in

the hydroisomerization of n-hexane is promoted both with the addition of Al to the ZrW mixed oxide and the addition of Pt to Pd/ZrAlW

catalysts. The improvement in the activity of Pd/ZrAlW catalysts is ascribed to a moderated acid strength and acidity, which can be correlated

to the coexistence of W6+ and reduced-state WOx species (either W4+ or W0). The addition of Pt to the Pd/ZrAlW catalyst does not modify

significantly its acidic character. Selectivity results showed that the catalyst produced 2MP, 3MP and the high octane 2,3-dimethylbutane (2,3-

DMB) and 2,2-dimethylbutane (2,2-DMB) isomers.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Bimetallic Pd–Pt catalysts; ZrO2–Al2O3–WO3 ternary oxide; Sol–gel method; Structure; Reducibility; XPS; Raman; NMR; Acidity;

Isomerization of n-hexane

1. Introduction

In recent years the catalytic processes to promote the

skeletal isomerization of light n-alkanes such as n-hexane

(n-C6H14) have played an important role in order to increase

the octane number of gasoline [1,2]. The industrial catalysts

used for the isomerization of alkanes are those based in Pt

supported on chlorinated alumina and Pt or Pd supported on

mordenite [1,2]. However, the former catalyst results in

serious environmental problems related to the recovery of

corrosive acids and it is sensitive to moisture and sulfur

impurities, while Pt or Pd supported on mordenite is less

* Corresponding author. Tel.: +52 55 91 75 8375; fax: +52 55 95 75 6239.

E-mail address: [email protected] (J.A. Montoya).

0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2005.05.011

active and works well at high temperatures with low yield of

branched isomers [3,4]. Arata et al. [5] have found that the

catalysts based on sulfated zirconia are highly active in the

hydroisomerization of alkanes at low temperatures, however

the inconvenient with these materials is that they are quickly

deactivated. It has also been reported that Pt catalysts

supported on zirconia promoted with tungsten are active in

the hydroisomerization of light n-alkanes at moderated

pressure and temperature [6–11]. However, the improve-

ment of their acid properties and hydrogenation capability is

highly desired in order to improve their catalytic properties

in the hydroisomerization of alkanes. On this respect, it is

known that the acidity of tungstated zirconia can be

increased by the addition of certain promoters as for

example Al [12–14]. Other noble metals than Pt, as for

A. Barrera et al. / Applied Catalysis A: General 290 (2005) 97–10998

example Pd with high hydrogenation capability, have been

scarcely studied [15,16]. An important aspect to consider is

the method of preparation of the catalytic materials. The

majority of Pt catalysts supported on zirconia promoted with

tungsten are prepared mainly by sequential impregnation of

zirconia with tungsten and Pt solutions to give Pt/WOx/ZrO2

systems [7–10,17]. However, the activity and selectivity of

the catalyst could depend of the form of interaction between

species which is a function of the method of preparation

used.

In this work, we have studied the activity and selectivity

in the hydroisomerization of n-hexane over mono- and

bimetallic Pd–Pt catalysts supported on ZrO2–Al2O3–WOx

oxides prepared by sol–gel via the hydrolysis of alkoxides

assisted by the aqueous ammonia metatungstate solution.

The activity test was carried out by applying high-

throughput reaction techniques. We have selected the more

active catalysts and tried to establish correlations between

their physicochemical properties and their catalytic perfor-

mance in the hydroisomerization of n-hexane.

2. Experimental

2.1. Catalysts preparation

70 wt.% ZrO2–15 wt.% Al2O3–15 wt.% WO3 (ZrAlW)

ternary oxides were prepared by the sol–gel method from

organic precursors with the hydrolysis process assisted by

the addition of the aqueous ammonia metatungstate solution

as follows: required volumes of zirconium(IV) propoxide,

Zr(O-Prop)4 (Aldrich), and aluminum-tri-sec-butoxide,

Al(O-sec-Bu)3 (Aldrich), were dissolved in a glass flask

containing 1-propanol (Fermont), and hexylene glycol, 2-

methyl-2,4-pentanediol (Aldrich), as a complexing agent

[18]. The solution was heated with continuous stirring to

70 8C and kept there for 1 h. Then, the required volume of

the aqueous ammonia metatungstate (AMT) solution

(NH4)6H2W12O40�nH2O (Aldrich) was added to the homo-

geneous alkoxide solution in the glass flask to perform the

hydrolysis. The gels obtained were aged in the glass flask

with the remaining AMT solution at 40 8C for 2 h and then at

room temperature for 12 h. After that, the gels were dried in

an oven at 100 8C for 12 h followed by calcination at 800 8Cfor 4 h. 85 wt.% ZrO2–15 wt.% WO3 (ZrW) binary oxides

were prepared by the same method except for the addition of

the alumina precursor.

The ZrW and ZrAlW oxides calcined at 800 8C were

impregnated with solutions of varying concentrations of

PdCl2 (Aldrich) to give catalysts with 0.3, 0.6, and 1.0 wt.%

of Pd. Some samples were coimpregnated with a

H2PtCl6�xH2O solution (Aldrich) to give bimetallic catalysts

with Pd (0.6 wt.%)–Pt (0.3 wt.%), and Pd (1.0 wt.%)–Pt

(0.3 wt.%). In order to study the effect of Pt, the ZrAlW

oxide was also impregnated with H2PtCl6�xH2O to give

catalysts with 0.3 and 0.9 wt.% Pt. After impregnation, the

samples were dried in two stages, firstly at 60 8C for 4 h and

then at 100 8C for 12 h. Finally the samples were calcined at

600 8C for 3 h.

2.2. Catalyst characterization

The surface area of Pd, Pt and Pd–Pt catalysts after

calcination at 600 8C for 3 h was determined by nitrogen

physisorption in an Autosorb gas sorption system (from

Quantachrome) on the basis of the BET equation from the

nitrogen isotherms. Before the measurements, the samples

were flushed with Ar at 200 8C for 2 h.

X-ray diffraction patterns of the catalysts calcined at

600 8C for 3 h were obtained in a Bruker-Axs D8 Discover

with GADDS (General Area Detector Diffraction Systems,

two-dimensional (2D) detector) diffractometer fitted with a

Cu anode tube (40 kV, 40 mA) by using the combinatorial

approach for both measurements and patterns evaluation

with the sample holder designed for 48 samples in one plate.

In this equipment the area detector is situated at 15 cm from

the sample and the step size is 0.058 with a scan time of

300 s.

Temperature-programmed reduction (TPR) profiles of

the catalysts calcined at 600 8C for 3 h were obtained under

hydrogen flow (5% H2/Ar) by using a thermodesorption

apparatus, Multipulse RIG model (from ISRI), equipped

with a thermal conductivity detector (TCD). Samples of

100 mg and a gas flow rate of 30 cm3 min�1 were used in the

experiments. The TPR profiles were registered by heating

the samples from 25 to 1000 8C at a rate of 10 8C min�1 and

the rate of hydrogen consumption was monitored by TCD.

The amount of hydrogen consumed was obtained by the

integration of the TPR profiles peaks with the standard

procedure included in the software of the Multipulse RIG

unit. This procedure presents splitting for unresolved peaks

with a vertical line dropped to the base line. Hydrogen pulses

were used to calibrate the TPR signal with the same flow

used in the TPR experiments.

XPS spectra of mono- and bimetallic Pd–Pt catalysts after

calcination were recorded on a VG ESCALAB 250

apparatus after excitation with a monochromatic Al Ka

radiation (hn = 1486.6 eV). Samples were embedded onto

Indium foils which were pressed manually and placed to a

holder. Calibration of the energy position of an XPS peak

was performed by using the binding energy of carbon 1s

peak at 284.8 eV. The unresolved XP spectra for the W 4f

transition were deconvoluted with three doublets by using a

standard Gaussian function. A binding energy splitting of

2.1 � 0.1 eV was adopted for the W 4f7/2–W 4f5/2

separation. The binding energies values were assigned to

the corresponding oxidation state according to literature

data.

Raman spectra of the mono- and bimetallic Pd–Pt

catalysts were recorded in the 100–1400 cm�1 wavenumber

range using a ThermoNicolet Raman apparatus (Almega

model) equipped with a Nd:YVO4DPSS laser source. The

A. Barrera et al. / Applied Catalysis A: General 290 (2005) 97–109 99

excitation line of the laser was 532 nm and the laser power

was of 25 mW.27Al NMR–MAS spectra of the catalysts calcined at

600 8C for 3 h were recorded at 23 kHz with a Bruker 400

spectrometer and using AlCl3 0.1 M solution as the standard

reference. Scan conditions were 0.35 ms pulse with a delay

time of 1.0 s. The unresolved 27Al NMR spectra were

deconvoluted in three peaks by using a standard Lorentzian

function.

FT-IR of adsorbed pyridine experiments were performed

in order to determine the type and amount of surface acid

sites of the ZrW mixed oxide and catalysts with 1.0 wt.% of

palladium. They were carried out by using a Fourier

transform infrared (FT-IR) Perkin-Elmer spectrometer

Model 170-SX. Pretreatment of the samples prior to the

adsorption of pyridine consisted in outgassing followed by

heating to 500 8C at 20 8C min�1 and cooling to room

temperature. After the pretreatment, the samples were

exposed to saturated pyridine vapor for 20 min. IR spectra

were recorded after desorption at 50, 100, 200, 300, and

400 8C.

2.3. Catalytic activity test

Hydroisomerization of n-hexane (n-C6H14) over the Pd,

Pt and Pd–Pt catalysts calcined at 600 8C for 3 h was

performed in a Multi-Channel Fixed Bed Reactor (Symyx).

This equipment is appropriate to evaluate the activity and

selectivity of 48 samples in parallel by applying high-

throughput testing techniques with a matrix arrangement of

6 columns � 8 rows. Briefly, this system consists of six

reactor heads each one containing eight wells for steel

micro-reactors of approximately 4 mm inner diameter and

47 mm length. The six reactor heads are connected

independently to six chromatographs (Agilent, 6850 Series)

equipped with a SPB-1 capillary column (Supelco) and a

flame ionization detector (FID) to do the reaction product

analysis. For the catalytic evaluation 100 mg of catalyst was

diluted with 200 mg of inert silicon carbide by mixing them

and then packed within the micro-reactors and fixed into the

well reactor heads. Pretreatment of the catalysts was carried

out in situ prior to the activity test, and consisted in a drying-

reduction program; drying the samples at 260 8C for 2 h in

helium (200 cm3 min�1) followed by reduction in hydrogen

flow (200 cm3 min�1) at 350 8C for 3 h.

Table 1

Specific surface area and H2 uptake from the TPR profiles of mono- and bime

calcination of the supports at 800 8C/4 h

Catalysts SBET (m2 g�1)

Pd (0.6 wt.%)/ZrW 69

Pd (0.6 wt.%)/ZrAlW 89

Pd (0.6 wt.%)–Pt (0.3 wt.%)/ZrAlW 80

Pt (0.3 wt.%)/ZrAlW 85

Pd (1.0 wt.%)/ZrW 67

Pd (1.0 wt.%)/ZrAlW 78

Pd (1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW 87

The experimental conditions in the hydroisomerization of

n-hexane were as follows: reaction pressure of 0.689 MPa,

reaction temperature of 220, 240 and 260 8C, the total feed

composition was a mixture of H2 = 0.269 mol/h,

He = 0.135 mol/h, and a liquid flow of n-hexane = 0.184 -

mol/h, giving a WHSV = 3.695 for every single well. The

H2/n-C6 molar ratio was 1.469, which is very close to that

used in the industrial process.

The catalytic evaluation procedure is fully automatized,

and for every run the pressure, reactor and accessory

temperature and reactant flows are set and controlled in real

time. When the experimental conditions are reached the first

row of six reaction products are analyzed simultaneously in

the six GC’s, the time for analysis method is 10.7 min, and

after the next injection proceeds, and so on until the eight

rows are analyzed. The 48 chromatograms are saved in a

database. In this way every six catalysts row are evaluated

simultaneously and at the same reaction time but the next

one has 10.7 min more of the reaction time. Therefore, the

total time for one run experiment is about 90 min. To

estimate if the deactivation process is important the catalysts

are evaluated in a different well position. In this particular

type of catalyst it was found that deactivation by carbon

deposition was not important at least during the run

experiment.

3. Results

3.1. BET surface area

The BET surface area (SBET) of the Pd, Pt, and bimetallic

Pd–Pt catalysts supported on ZrW and ZrAlW prepared by

sol–gel via the hydrolysis assisted by the AMT solution is

shown in Table 1. Their SBET were in the range 67–

89 m2 g�1 with the lowest values observed in the catalysts

supported on ZrW (alumina free). The addition of 15 wt.%

of alumina increased the SBET of the catalysts for about 15%.

3.2. X-ray diffraction

The XRD patterns of the Pd and bimetallic Pd–Pt

catalysts supported on ZrAlW after calcination at 600 8C are

shown in Fig. 1. Pd, Pt or Pd–Pt phases were not observed.

For the catalysts supported on ZrW (0 wt.% of Al2O3),

tallic Pd–Pt/ZrO2–Al2O3–WOx catalysts calcined at 600 8C/3 h; previous

H2 uptake of the TPR peak at T = 80 8C (mmol g�1)

11.28

31.17

35.97

–

69.00

38.60

27.60


Fig. 1. XRD patterns of mono- and bimetallic Pd–Pt/ZrO2–Al2O3–WOx

catalysts calcined at 600 8C/3 h; previous calcination of supports at 800 8C/

4 h: (a) ZrW; (b) Pd (0.6 wt.%)/ZrW; (c) Pd (0.6 wt.%)/ZrAlW; (d) Pd

(0.6 wt.%)–Pt (0.3 wt.%)/ZrW; (e) Pd (0.6 wt.%)–Pt (0.3 wt.%)/ZrAlW; (f)

Pt (0.3 wt.%)/ZrAlW; (g) Pd (0.9 wt.%)/ZrAlW; (h) Pd (1.0 wt.%)/ZrW; (i)

Pd (1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW.

Fig. 2. TPR profiles of the ZrW support and the mono- and bimetallic Pd

(0.6 wt.%)–Pt (0.3 wt.%)/ZrO2–Al2O3–WOx catalysts. The catalysts were

calcined at 600 8C/3 h; previous calcination of the supports at 800 8C/4 h.

diffraction lines at 2u = 23.38, 23.88 and 24.58 assigned to

WO3 in the monoclinic phase (75-2072 JSPDS card) were

detected along with zirconia in the tetragonal phase (79-

1769 JPDS card). In contrast, for the catalysts supported on

ZrAlW (15 wt.% of Al2O3), the WO3 phase was not

observed, which suggests that the Al2O3 enhances the WO3

dispersion, and only diffraction lines belonging to ZrO2 in

the tetragonal phase were detected. In addition, the

incorporation of Al2O3 affects the crystallite size of the

tetragonal ZrO2 which decreases from 12.0 nm for samples

without alumina to 7.0 nm for the catalysts supported on

ZrAlW.

3.3. Temperature-programmed reduction

Fig. 2 shows the TPR profiles for the catalyst series: ZrW,

Pd (0.6 wt.%)/ZrW, Pd (0.6 wt.%)/ZrAlW, Pd (0.6 wt.%)–Pt

(0.3 wt.%)/ZrAlW, Pt (0.3 wt.%)/ZrAlW, and Pt (0.9 wt.%)/

ZrAlW. The TPR profile of the ZrW oxide shows a small

reduction peak centered at ca. 470 8C, a broad shoulder at

temperatures higher than 600 8C which is overlapped with an

intense reduction peak centered at ca. 950 8C as showed by the

dot lines of the deconvoluted TPR profile. The small reduction

peak at ca. 470 8C also appears in Pd (0.6 wt.%)/ZrW,

however it disappears in the catalysts supported on ZrAlW.

The TPR profiles of the mono-Pd, Pt and bimetallic Pd–Pt

catalysts also show the broad shoulder at temperatures higher

than 600 8C that is overlapped with the more intense reduction

peak at ca. 950 8C. However, this broad shoulder is shifted to

higher temperatures for about 40 8C in the TPR profiles of Pd

(0.6 wt.%) and Pd (0.6 wt.%)–Pt (0.3 wt.%) catalysts

supported on ZrAlW with respect to that of Pd (0.6 wt.%)/

ZrW. Moreover, the TPR profiles of the catalysts with

0.6 wt.% of palladium show the appearance of a reduction

peak at approximately 80 8C which is not observed in Pt

(0.3 wt.%)/ZrAlW or Pt (0.9 wt.%)/ZrAlW, and increases its

intensity in the sequence: Pd (0.6 wt.%)/ZrW < Pd

(0.6 wt.%)/ZrAlW < Pd (0.6 wt.%)–Pt (0.3 wt.%)/ZrAlW.

The hydrogen consumption for this reduction peak is shown

in Table 1.

3.4. XPS measurements

Figs. 3–5 show the high resolution XP spectra of the W 4f

transition which allows the detailed analysis of the W

chemical state and the interaction with the elements that

compose the mono- and bimetallic Pd–Pt catalytic system. It

can be observed in the 30–42 eV region a complex band

(quadruplet) with a gap between the maximums of 2.1 eV.

This gap is characteristic of oxidized states of Wand belongs

to the spin–orbital coupling generating the 4f7/2 and 4f5/2

transitions. The XPS W 4f spectra are fitted by three


Fig. 3. XP spectra of the W 4f region of the Pd (0.6 wt.%)/ZrAlW catalyst

calcined at 600 8C/3 h; previous calcination of the support at 800 8C/4 h.

Inside: XP spectra of the W 4f region of the WO3 standard.

Fig. 4. XP spectra of the W 4f region of the Pd (0.6 wt.%)–Pt (0.3 wt.%)/

ZrAlW catalyst calcined at 600 8C/3 h; previous calcination of the support

at 800 8C/4 h.

Fig. 5. XP spectra of the W 4f region of the Pt (0.3 wt.%)/ZrAlW catalyst

calcined at 600 8C/3 h; previous calcination of the support at 800 8C/4 h.

doublets corresponding to three different oxidation states as

indicated by the overlapped individual peaks obtained after

deconvolution. The binding energies (BE) belonging to the

W 4f7/2 transition of the mono- and bimetallic Pd–Pt

catalysts are shown in Table 2. The BE depends on the

composition of the catalyst and the small dispersion of the

value obtained after fitting is related to the presence of a

mixture of tungsten oxide. It has been reported that the

mixture of oxides (WO2 and WO3) for a system modifies

slightly the BE of W with respect to the values obtained from

the pure oxides [19]. This suggests the presence of a

complex mixture of chemical states of W on the surface of

these catalytic materials. The first doublet comprises BE for

the W 4f7/2 transition in the 31.97–34.18 eV range. The BE

of 31.97 eV could be assigned to W0 (BE = 32.0 eV) [20].

Table 2

Binding energies (eV) of W 4f7/2 transition, atomic W/(Zr + Al) ratio, and surface

600 8C/3 h; previous calcination of the mixed oxide supports at 800 8C/4 h

Catalyst Binding energy (eV), W 4f7/2

Pd (0.6 wt.%)/ZrAlW 33.00 36.03

Pd (0.6 wt.%)–Pt (0.3 wt.%)/ZrAlW 31.97 36.44

Pd (1.0 wt.%)/ZrW 34.18 35.69

Pd (1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW 33.59 35.12

Pt (0.3 wt.%)/ZrAlW 33.42 34.93a Atomic W/Zr ratio.

The BE between 33.0 and 33.59 eV can be assigned to W in

a oxidation state of W4+ [20,21–25]. Whereas the BE of

34.18 eV is close to the BE value of 34.2 eVattributed to the

presence of W5+ surface species [26].

The doublet shifted to higher energy (4f7/2 at about

34.93–36.44 eV) can be assigned to either W6+ (WO3) or

Al2(WO4)3 [26–28]. The third doublet (W 4f7/2 in the 37.51–

38.68 eV range) has a large shift that can only be explained

by the presence of a highly electronegative element (Cl�1)

interacting strongly with the tungsten [29,30].

3.5. Raman spectra

Raman spectra of the ZrAlW mixed oxide and Pd

(1.0 wt.%) catalysts is shown in Fig. 6. The spectra show the

presence of a band at about 652 cm�1 that is assigned to the

tetragonal ZrO2 phase [8]. The spectra also includes a band

at 926 cm�1 which is assigned to W O stretching mode of

tungstate monomeric species in tetrahedral coordination

[15]. The sharp band at about 1013 cm�1 corresponds to the

W O stretching mode which is consistent with the presence

of mono-oxotungstate species [8].

3.6. 27Al NMR–MAS spectra

Fig. 7 shows the 27Al NMR spectra of the mono and

bimetallic Pd–Pt catalysts supported on ZrAlW. Each

spectrum contains a main intense resonance line (chemical

shift) in the �2.32 to 1.51 ppm region assigned to an Al3+

species in octahedral coordination (Fig. 7, P1). It is known

density of W atoms of the mono- and bimetallic Pd–Pt catalysts calcined at

Atomic W/(Zr + Al) ratio Surface density of

W atoms (W/nm2)

38.43 0.115 4.4

37.51 0.199 4.9

39.08 0.191a 5.8

38.27 0.173 4.5

38.68 0.171 4.6


Fig. 6. Raman spectra of ZrAlW mixed oxide calcined at 800 8C/4 h and

catalysts with 1.0 wt.% of palladium calcined at 600 8C/3 h; previous

calcination of the supports at 800 8C/4 h: (a) ZrAlW; (b) Pd (1.0 wt.%)/

ZrAlW; (c) Pd (1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW; (d) Pd (1.0 wt.%)/ZrW.

that the octahedral Al3+ appears at 0 ppm for the a-Al2O3, so

that the chemical shift observed for 27Al (�2.32 to

1.51 ppm) in the ZrAlW catalysts indicates a distorted

octahedral coordination. In addition, all the spectra show

asymmetry suggesting the presence of Al3+ species in

different chemical environments. The deconvoluted spectra

contain two shoulders, a small shoulder is observed in the

Fig. 7. 27Al NMR–MAS spectra of the mono- and bimetallic Pd

(0.6 wt.%)–Pt (0.3 wt.%)/ZrO2–Al2O3–WOx catalysts calcined at 600 8C/

3 h; previous calcination of the supports at 800 8C/4 h. (P1) chemical shift in

the �2.32 to 1.51 ppm region; (P2) chemical shift in the 22.19–27.10 ppm

region; (P3) chemical shift in the 54.90–64.62 ppm region.

22.19–27.10 ppm region that can be attributed to the

presence of pentacoordinated AlV ions (Fig. 7, P2) and a

broad shoulder in the range 54.9–64.62 ppm assigned to

tetrahedral AlIV species (Fig. 7, P3).

3.7. IR of adsorbed pyridine

Fig. 8 shows the IR spectra of adsorbed pyridine of the ZrW

mixed oxide and the Pd (1.0 wt.%)/ZrW, Pd (1.0 wt.%)/

ZrAlW, Pd (1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW catalysts after

outgassing increasing temperatures. The IR spectra of the

catalysts contain characteristic bands of pyridine coordina-

tively bonded to Lewis acid sites at about 1443–1460 and

1490–1500 cm�1, whereas the bands at about 1540 cm�1 are

assigned to protonation of pyridine by Bronsted sites. The

intensity of these bands is retained after rising the temperature

up to 400 8C in the IR spectra of Pd (1.0 wt.%)/ZrW whereas

those of Pd (1.0 wt.%)/ZrAlW and Pd (1.0 wt.%)–Pt (0.3%)/

ZrAlW are almost completely removed at 300 8C. Fig. 9

shows that the acidity of the catalysts varied in the order:

ZrW < Pd (1.0 wt.%)/ZrW and Pd (1.0 wt.%)/ZrW > Pd

(1.0 wt.%)/ZrAlW � Pd (1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW.

3.8. Activity test; the isomerization of n-hexane

3.8.1. Catalytic activity at different temperatures over

the monometallic catalysts

The conversion (%) in the hydroisomerization of n-

hexane for the monometallic Pd and Pt catalysts at different

reaction temperatures are depicted in Fig. 10. By comparing

the n-C6H14 conversion (%) of the mono metallic catalysts,

Pd (0.3 wt.%)/ZrAlW catalyst (Fig. 10b) with that of Pt

(0.3 wt.%)/ZrAlW (Fig. 10a), it is observed that the Pd

catalyst developed considerable higher activity than the Pt

catalysts in the whole temperature range. At 220 8C, the n-

hexane conversion of the catalysts was in the range 12–17%.

At 240 8C, the n-C6H14 conversion over the Pd catalysts

increased to 19–36% with the less active one being the Pd

(0.6 wt.%)/ZrW and the more active one being the Pd

(0.6 wt.%)/ZrAlW, Pd (1.0 wt.%)/ZrW and Pd (1.0 wt.%)/

ZrAlW catalysts (Fig. 10d–f). At 260 8C, the activity for the

Pd catalysts increased to 39–57% and the increase in the

activity by the addition of alumina becomes more clear as

well as the increase in the activity with the concentration of

palladium.

3.8.2. Catalytic activity over the mono- and bimetallic

Pd (x = 0.6, 1.0 wt.%)–Pt (0.3 wt.%) catalysts

Fig. 11 shows the n-hexane conversion (%) at 260 8C for

a series of mono- and bimetallic Pd (x)–Pt (0.3 wt.%)

catalysts (x = 0.6 wt.% (Fig. 11a–c), 1.0 wt.% (Fig. 11f–

h)). In order to study the effect of Pt in the catalysts, the n-

hexane conversion % over the catalysts with Pt (0.3,

0.9 wt.%) supported on ZrAlW are also depicted (Fig. 11d

and e). It can be observed that the n-hexane conversion in

the Pd (0.6 wt.%) catalysts increases in the order: Pd/ZrW


Fig. 8. FT-IR spectra of adsorbed pyridine of the ZrW mixed oxide calcined at 800 8C/4 h and the catalysts with 1.0 wt.% of palladium calcined at 600 8C/3 h;

previous calcination of the supports at 800 8C/4 h.

(�40%) < Pd/ZrAlW (�50%) < Pd–Pt/ZrAlW (�55%)

(Fig. 11a–c). The same behavior is seen for the series of

catalysts with Pd (1.0 wt.%) (Fig. 11f–h), and their activity

is higher than the activity of Pd (0.6 wt.%) catalysts, with

the n-hexane conversion % increasing in the sequence: Pd/

ZrW (50%) < Pd/ZrAlW (57%) < Pd–Pt/ZrAlW (61%).

On the other hand, the activity of Pt (0.3 wt.%)/ZrAlW

(Fig. 11d) is small (�5%) but the addition of Pt to the Pd

catalysts contributes to increase the activity for about 9%.

Lower activity is also observed when a Pt loading

equivalent to 0.9 wt.% is tested (Fig. 11e); its n-hexane

conversion (�26%) is even lower than that of the Pd

(0.6 wt.%)/ZrW.

Fig. 12 shows the selectivity % in the hydroisomerization

of n-hexane at 260 8C over the catalysts. It can be observed

that 2-methylpentane (2-MP) is the main product (50%) for

all the catalysts followed by 3-methylpentane (3-MP)

(�35%), 2,3-dimethylbutane (2,3-DMB) (�10%) and 2,2-

dimethylbutane (2,2-DMB) (�5%). The formation of a

slight amount of light products is only observed with the Pd

catalysts supported on ZrW.

4. Discussion

The enhancement of SBET of Pd/ZrAlW catalysts after

calcination at 600 8C for 3 h with respect to that of Pd/ZrW

catalysts indicates that Al2O3 acts as a textural promoter of

the ZrW oxide. Besides, the high SBET of the catalysts

prepared by sol–gel via the hydrolysis assisted by the

aqueous AMT solution is due to the high homogenization of

the components in the material. This is supported by the fact

that there are no diffraction lines of alumina in the patterns

indicating that alumina was homogenously mixed with

zirconia and WOx. The fact that Pd, Pt or Pd–Pt phases were

no detected by XRD (Fig. 1) suggests that oxidized particles

were either non-crystalline or very small. The detection of a

slight amount of the monoclinic WO3 phase in the catalysts

supported on ZrW suggests that zirconia facilitates the

formation of some WO3 microcrystallites. For the catalysts

supported on ZrAlW, only diffraction lines belonging to the

tetragonal zirconia phase were detected indicating that WOx

is highly dispersed both on the surface as within the bulk of

the oxide support either as amorphous or microcrystalline


Fig. 9. Total acidity and amounts of Bronsted acid and Lewis acid of the ZrW mixed oxide calcined at 800 8C/4 h and the catalysts with 1.0 wt.% of palladium

calcined at 600 8C/3 h; previous calcination of the supports at 800 8C/4 h.

particles which are no detected by XRD. It is interesting to

note that the diffraction lines at 2u = 50.58 belonging to

tetragonal zirconia were more intense and sharp in the

Fig. 10. Conversion % vs. reaction temperature in the hydroisomerization

of n-hexane over the monometallic catalysts reduced in H2 at 350 8C/3 h

after calcination of the catalysts at 600 8C/3 h; previous calcination of the

supports at 800 8C/4 h: (a) Pt (0.3 wt.%)/ZrAlW; (b) Pd (0.3 wt.%)/ZrAlW;

(c) Pd (0.6 wt.%)/ZrW; (d) Pd (0.6 wt.%)/ZrAlW; (e) Pd (1.0 wt.%)/ZrW;

(f) Pd (1.0 wt.%)/ZrAlW.

catalysts supported on ZrW indicating a high crystallinity of

zirconia and thereby the presence of bigger particles. This

suggests that the incorporation of Al into the ZrW mixed

oxide modifies the crystallinity of the material and the

zirconia particles become smaller.

The small reduction peak at ca. 470 8C in the TPR

profiles of ZrW and Pd/ZrW samples (Fig. 2) is assigned to

the reduction of some WO3 microcrystallites [31–33]. The

Fig. 11. Conversion % in the hydroisomerization of n-hexane at 260 8Cover the mono- and bimetallic Pd–Pt catalysts reduced in H2 at 350 8C/3 h

after calcination of the catalysts at 600 8C/3 h; previous calcination of the

supports at 800 8C/4 h: (a) Pd (0.6 wt.%)/ZrW; (b) Pd (0.6 wt.%)/ZrAlW;

(c) Pd (0.6 wt.%)–Pt (0.3 wt.%)/ZrAlW; (d) Pt (0.3 wt.%)/ZrAlW; (e) Pt

(0.9 wt.%)/ZrAlW; (f) Pd (1.0 wt.%)/ZrW; (g) Pd (1.0 wt)/ZrAlW; (h) Pd

(1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW.


Fig. 12. Selectivity % in the hydroisomerization of n-hexane at 260 8C over

the mono- and bimetallic Pd–Pt catalysts reduced in H2 at 350 8C/3 h after

calcination of the catalysts at 600 8C/3 h; previous calcination of the

supports at 800 8C/4 h: (a) Pt (0.3 wt.%)/ZrAlW; (b) Pd (0.3 wt.%)/ZrAlW;

(c) Pd (0.6 wt.%)/ZrW; (d) Pd (0.6 wt.%)/ZrAlW; (e) Pd (0.6 wt.%)–Pt

(0.3 wt.%)/ZrAlW; (f) Pd (1.0 wt.%)/ZrW; (g) Pd (1.0 wt.%)/ZrAlW; (h) Pd

(1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW.

disappearance of this reduction peak in the TPR profiles of

the catalysts supported on ZrAlW suggests that the addition

of Al to the ZrW mixed oxide support prevent the formation

of WO3 microcrystallites. The broad shoulder observed

between 600 and 900 8C in the TPR profiles is assigned to

the reduction of dimeric tungstate species in tetrahedral

coordination [31,34], whereas the intense reduction peak

observed at temperatures higher than 800 8C corresponds to

the reduction of isolated monomeric tungstate species in

tetrahedral coordination which are strongly bound to ZrO2

and Al2O3 [31,34]. No reduction peak attributed to the

presence of octahedral tungstate species was detected in the

TPR profiles. Therefore, only monomeric and dimeric

tungstate species in tetrahedral coordination are present in

the catalysts as inferred by the inflection point separating the

overlapped reduction peaks. The shifting in the reduction

temperature for about 40 8C higher in the catalysts supported

on ZrAlW indicates that Al3+ interacts strongly with the

tetrahedrally coordinated tungstate species retarding their

reduction. Since no reduction peaks are observed at

temperatures lower than 200 8C in the Pt/ZrAlW catalysts,

it is deduced that platinum oxide species reduce at room

temperature or lower and that there is a negligible interaction

with the tungstate species. However, the reduction peak at

ca. 80 8C in the TPR profiles of the samples is observed only

in the catalysts containing Pd, therefore this peak must be

assigned to the reduction of PdO species. This reduction

peak is observed for about 45 8C higher than that reported

for the reduction of PdO supported on zirconia [35]

suggesting the existence of a strong interaction between PdO

and the surface WOx species of the mixed oxide supports.

The variation of PdO reducibility at ca. 80 8C (Table 1) in the

sequence Pd (0.6 wt.%)/ZrW < Pd (0.6 wt.%)/ZrAlW < Pd

(0.6 wt.%)–Pt (0.3 wt.%)/ZrAlW, and Pd (1.0 wt.%)/

ZrW > Pd (1.0 wt.%)/ZrAlW > Pd (1.0 wt.%)–Pt (0.3

wt.%)/ZrAlW could be related to the interaction extent

between PdO and the surface WOx species since the atomic

W/(Zr + Al) ratio (Table 2) in the catalysts varied in the

same way as the H2 uptake, i.e. the atomic ratio of tungsten

increases in the Pd (0.6 wt.%) catalysts whereas it decreases

in the Pd (1.0 wt.%) catalysts. This observation suggests that

the variation in the reducibility of PdO could depend on both

the load of palladium as well as the amount of surface WOx

species interacting with PdO. The hydrogen uptake for this

reduction peak corresponds to less than 100% of reduction

of PdO species. Thereby, it is assumed that an important

amount of PdO species are already reduced at room

temperature or lower, which is evidenced by the small

negative peak at ca. 70 8C in the TPR profile of Pd

(0.6 wt.%)/ZrW indicating the decomposition of palladium

hydride which is formed upon absorption of hydrogen on

Pd0 [36]. This observation agrees with the references that

report that a high amount of metallic palladium is already

present in Pd/WO3 and Pd/WO3/Al2O3 samples before any

hydrogen treatment which was ascribed to an autoreduction

of the metallic precursor containing chlorine [37].

The presence of the surface WOx species is corroborated

from the W 4f XP spectra of the catalysts calcined at 600 8C/

3 h (Figs. 3–5). However, a striking result is the coexistence

of various oxidation states as observed after deconvolution

of the XP spectra. The lower BE of 31.97 eVobserved in the

first component of the W 4f XP spectra of the bimetallic Pd

(0.6 wt.%)–Pt (0.3 wt.%)/ZrAlW catalyst (Table 2) is close

to the BE value of 32.0 eV assigned to the presence of W0

[20]. However, this BE could correspond to the so-called b-

phase (W3O) which is metastable and it is almost impossible

to distinguish from metallic tungsten by XPS [37]. The

maximum BE value for the first component of W 4f XP

spectra is observed in the Pd (1.0 wt.%)/ZrW catalyst and

corresponds to the presence of W5+ species [20–24,26].

Whereas, the BE between 33.0 and 33.59 eVobserved in the

remainder catalysts are in the range corresponding to W4+

[20–24,26].

Concerning the presence of reduced-state WOx species in

the calcined samples, it has been suggested that a

condensation phenomenon of surface monomeric tungstate

species leading to polymeric species could explain the

formation of W5+ in WO3/ZrO2 [26]. It is also known that the

reduction of W6+–W4+ can occur by injection of trapped

electrons into the WO3 lattice [39]. In our catalysts a high

amount of Pd0 and Pt0 is already present before any

hydrogen treatment, then, an electronic transfer from the

noble metal to the surface WOx species could occur due to

interaction effects [40]. Moreover, the reduction of WOx

species could be facilitated by the delocalization of the

negative charge onto the WOx network [41], promoted in our

opinion by the attractive potential of coordinative unsatu-

rated Zr4+ and Al3+ ions (Lewis acid sites) of the Zr–O–W

and Zr–O–Al–O–W mixed oxides.


The BE in the range 36.03 and 36.44 eV observed in the

second component of the W 4f XP spectra (Table 2) are close

to the values assigned to hexavalent tungsten compounds

(W6+) corresponding to either WO3 or Al2(WO4)3 species

[26–28]. Since Al3+ interacts strongly with the tetrahedrally

coordinated tungstate species in Pd (0.6 wt.%) catalysts

supported on ZrAlW, as inferred from their TPR profiles

(Fig. 2), it is assumed that these BE must correspond to the

presence of Al2(WO4)3 species. Whereas, the BE of

35.69 eV observed in the W 4f XP spectra of Pd

(1.0 wt.%)/ZrW is close to the BE value of 35.7 eV assigned

to W6+ species [11,27], confirming the formation of some

WO3 microcrystallites on the surface of Pd/ZrW catalysts.

The BE of 34.93 and 35.12 eV observed in the W 4f XP

spectra of Pt (0.3 wt.%)/ZrAlW and Pd (1.0 wt.%)–Pt

(0.3 wt.%)/ZrAlW catalysts are slightly lower than the BE

value of 35.2 eV (W6+) of the standard WO3.

The BE (37.51–38.68 eV) observed in the third

component of the XP spectra are higher than those reported

of WO2Cl2 (37.1 eV) or WOCl4 (37.4 eV) [30]. These high

BE are due to the presence of tungsten compounds having

highly electronegative ligands [38]. It is known that residual

chlorine is remaining when WO3 is impregnated with

metallic precursors containing chlorine even after calcina-

tion of the catalysts at 700 8C, the relative amount of which

depends on the WO3 loading [23]. Since the ZrWand ZrAlW

mixed oxide supports were impregnated from PdCl2 and

H2PtCl6�xH2O, then residual chlorine could be located on

tungsten oxide to form WxOyCl compounds [37].

Regarding the structure of the surface WOx species,

Raman spectra show that isolated monomeric tungstate

species in tetrahedral coordination ([WO4] units) are present

in the ZrAlW mixed oxide and Pd (1.0 wt.%) catalysts as

inferred by the band at about 926 cm�1 (W O stretching

mode [15]) (Fig. 6). The existence of two-dimensional

mono-oxotungstate species is also deduced from the band at

1013 cm�1 (W O stretching mode) [8,41–43]. According to

Iglesia et al. [41], these terminal W O bonds are common in

well-dispersed WOx domains which are observed at tungsten

surface coverage lower than the theoretical monolayer

(�7 W atoms nm�2). Besides, no Raman bands at about

300, 600, 807, and 830 cm�1 characteristics of W–O–W

linkages [8,15,31,41–43] or 270, 715, and 800 cm�1

ascribed to bulk WO3 [8,15,43] are present in the catalysts.

Table 327Al NMR–MAS spectra of mono- and bimetallic Pd–Pt catalysts calcined at 60

Catalysts Population %

P1, octahedral (�2.32 to 1.51 ppm)

Pd (0.6 wt.%)/ZrW –

Pd (0.6 wt.%)/ZrAlW 90.47

Pd (0.6 wt.%)–Pt (0.3 wt.%)/ZrAlW 86.71

Pt (0.3 wt.%)/ZrAlW 88.28

Pd (1.0 wt.%)/ZrW –

Pd (l.0 wt.%)/ZrAlW 64.10

Pd (1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW 70.39

This observation suggests the lack of W–O–W connectivity

and confirms that the WOx species are highly dispersed on

the surface of the mixed oxides. In our catalysts, the

theoretical WOx surface coverage (Table 2) for 15 wt.% of

tungsten oxide is slightly below of that predicted for a two-

dimensional polytungstate monolayer (�7 W atoms nm�2)

[41], which is due to the high dispersion of WOx species as

on the surface as within the bulk of the mixed oxide

supports. It is well known that the formation of octahedrally

coordinated tungstate species requires WOx surface cov-

erages higher than a monolayer [26]. Then, it is suggested

that the structure of the highly dispersed WOx species in

these catalysts consists mainly of oxotungstate species in

which the tungsten is in tetrahedral coordination form. This

is corroborated from their TPR profiles because there is no

reduction peaks assigned to octahedrally coordinated WOx

species, and monomeric and dimeric WOx species in

tetrahedral coordination are predominant in the catalysts

(Fig. 2). Besides, the reduction of a slight amount of WO3

microcrystallites is observed only in the Pd/ZrW catalysts

having the higher WOx surface coverage (Fig. 2, Table 2).

Therefore, we can conclude on the basis of TPR and Raman

experiments that the structure of the surface WOx species in

the catalysts consists of oxotungstate species which are

constituted mainly of monomeric and dimeric species in

tetrahedral coordination.

The incorporation of Al within the Zr–O–W mixed oxide

framework prepared by sol–gel is suggested from the 27Al

NMR spectra of the mono- and bimetallic Pd–Pt catalysts

supported on ZrAlW (Fig. 7, Table 3) by the resonance line

observed in the 54.9–64.6 ppm region (P3) attributed to AlIV

species in tetrahedral coordination [12,44–46]. The higher

amount of tetrahedral AlIV species in the catalysts with

1.0 wt.% of palladium could be due to a high redispersion of

Al atoms caused by a higher redissolution of alumina

aggregates or to a higher transformation of the octahedral

AlVI species during calcination. The presence of bulk

alumina aggregates is inferred from the main intense

resonance line in the �1.7 to 1.1 ppm region (P1) of the 27Al

NMR spectra which is associated to AlVI species in an

octahedral coordination and represent AlVI�O� units [44].

The transformation of AlVI species into AlIV species is

corroborated by the presence of unsaturated aluminum ions

in octahedral position as deduced by the resonance line

0 8C/3 h: population % of each deconvoluted spectral component

P2, pentahedral (26.3–35.17 ppm) P3, tetrahedral (57.13–64.62 ppm)

– –

1.28 8.23

5.18 8.18

4.22 7.49

– –

9.84 26.00

9.17 20.40


observed in the 22.2–27.1 ppm region (P2) which is close to

the chemical shift produced by five oxygen atoms

surrounding an aluminum nucleus (AlV pentacoordinated

species) [45].

The activity of palladium catalysts in the hydroisome-

rization of n-hexane is promoted by both, the addition of Al

to the ZrW oxide support and the addition of Pt to the

catalyst. The addition of Al to the ZrW oxide improves the n-

C6H14 conversion in the palladium catalysts for about 25%,

whereas the addition of Pt to the catalysts contributes to

increase the activity for about 10% (Fig. 11). The lower

activity of the palladium catalysts supported on ZrW is

correlated with the presence of some surface WO3

microcrystallites as deduced from the BE of 35.69 eV

assigned to W6+ species (Table 2). However, these surface

W6+ species coexist with W5+ species (BE of 34.18 eV). The

presence of surface W5+ in Pd/ZrW catalysts could be

related to the formation of the nonstoichiometric W20O58

(WO2.59) phase which is constituted of W6+ and W5+ cations

[26,37,47]. Moreover, the existence of cracking leading to a

slight amount of light products as well as skeletal

isomerization to form 2MP, 3MP, 2,2DMB and 2,3DMB

in the Pd/ZrW catalysts (Fig. 12c and f) suggests the

participation of strong acid sites. This is corroborated by the

intensity of the bands at 1445, 1489 cm�1 (Lewis sites), and

1540 cm�1 (Bronsted sites) that is retained up to 400 8C in

the FT-IR spectra of adsorbed pyridine of the Pd (1.0 wt.%)/

ZrW catalyst, whereas those of ZrW, Pd (1.0 wt.%)/ZrAlW

and Pd (1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW are completely

removed at this temperature (Fig. 8), inferring therefore that

the acid strength of this catalyst was stronger. Besides, the

acidity of the Pd (1.0 wt.%)/ZrW catalyst was enhanced with

respect to that of the ZrW mixed oxide due to the increase in

the number of Lewis acidic sites (Fig. 9) although the

number of Bronsted acid sites was keep nearly constant.

Therefore, we assume that the strong acidic sites of the Pd/

ZrW catalysts would be generated by the coexistence of the

surface W6+ and W5+ species in intimate contact with

palladium.

Nevertheless, a striking result in our work is that the

improvement in the activity of the palladium catalysts

promoted by Al is correlated with a lower acid strength and

acidity than those of Pd/ZrW as observed from the

disappearance of the bands at 1448, 1490 cm�1 (Lewis

sites), and 1540 cm�1 (Bronsted sites) at 300 8C in the FT-IR

spectra of the Pd (1.0 wt.%)/ZrAlW catalyst (Fig. 8) and by

the decrease in the total amount of Bronsted and Lewis sites

of this catalyst (Fig. 9). The lower acid strength and the

decrease in the amount of acid sites of ZrAlW with respect to

that of ZrW is also confirmed from its FT-IR spectra of

adsorbed pyridine (figure not shown). The lower acidic

character of Pd/ZrAlW suggests that the nature of the

surface WOx species must be different than that in Pd/ZrW

catalysts. This is corroborated from the W 4f XP spectra of

the Pd (0.6 wt.%)/ZrAlW catalyst where W6+ cations (BE of

36.03 eV) corresponding to Al2(WO4)3 species coexist with

W4+ cations (BE of 33.0 eV) attributed to WO2 species and

from its Raman spectra showing a higher amount of mono-

oxotungstate species in tetrahedral coordination (Fig. 6).

Therefore, we suggest from the above-mentioned

considerations that the improvement in the activity of Pd/

ZrAlW catalysts with respect to that of Pd/ZrW is associated

to a lower acid strength and acidity of the catalyst, which can

be correlated to the coexistence between surface W6+

species and reduced-state WOx species (either W4+ or W0)

forming tetrahedrally coordinated oxotungstate species.

Another possibility of the decrease in the acidity of Pd/

ZrAlW catalysts could be due to the generation of Lewis

basic sites (coordinative unsaturated oxide ions of alumina)

during the calcination of the ZrAlW supports at 800 8C. It is

well known that the generation of Lewis basic sites in

alumina occurs easily by dehydration when a proton

bounded to an oxygen atom bridging octahedral and

tetrahedral aluminum atoms is combined with a basic

hydroxyl of an aluminum atom in octahedral coordination

[48]. Therefore, by the regular dehydration process of

alumina both AlV pentacoordinated atoms (Lewis acid sites)

and coordinative unsaturated (cus) oxygen atoms (Lewis

basic sites) are generated. In our catalysts a higher

population of AlV species (Lewis acid sites) is observed

in the 27Al NMR spectra of the Pd (1.0 wt.%)/ZrAlW and Pd

(1.0 wt.%)–Pt (0.3 wt.%)/ZrAlW (Table 3), which are the

more active catalysts. These catalysts have a lower acid

strength and amount of acid sites than that of Pd (1.0 wt.%)/

ZrW, therefore we infer that these catalysts should contain

also a high amount of cus oxide ions of alumina (Lewis basic

sites). Thereby, we believe that the lower acid strength and

the decrease in the amount of acid sites in Pd/ZrAlW could

be related to both the coexistence W6+ and reduced-state

WOx species (W4+ or W0) forming tetrahedrally coordinated

oxotungstate species and the presence of Lewis basic sites of

alumina. This assumption suggests a participation of both

Lewis acid sites (reduced-state WOx species and cus AlV

atoms) as well as Lewis basic sites (cus oxide ions of

alumina) on the skeletal isomerization of n-hexane which

must influence the catalytic selectivity. The lower acidic

character could explain the absence of cracking in the mono-

and bimetallic palladium catalysts supported on ZrAlW.

The coexistence between different oxidation states of

tungsten is also observed in the Pd (0.6 wt.%)–Pt (0.3 wt.%)/

ZrAlW catalyst where the surface W6+ cations (BE of

36.44 eV) assigned to Al2(WO4)3 species coexist with the

highly reduced tungsten oxide phase, W3O (BE of 31.97 eV)

which could be formed from the W20O58 phase [37,47]. The

same observation occurs with the most active Pd (1.0 wt.%)–

Pt (0.3 wt.%)/ZrAlW catalyst, where W6+ cations (BE of

35.12 eV) coexist with W4+ species (BE of 33.59 eV)

corresponding to WO2. Because the acidity of this catalyst

was similar to that of the Pd (1.0 wt.%)/ZrAlW, it is deduced

that neither the presence of a slightly lower amount of

pentacoordinate AlV species nor the addition of Pt to the Pd/

ZrAlW catalyst modifies significantly its acidic character.


Moreover, the improvement in the activity of the bimetallic

Pd–Pt catalysts is not only due to a higher load of metal as

confirmed by comparing the activity of the Pd (0.6 wt.%)–Pt

(0.3 wt.%)/ZrAlW with that of Pt (0.9 wt.%)/ZrAlW having

the same load of noble metal (Fig. 11c and e), since the n-

hexane conversion of the last catalyst was nearly the half of

the former. Therefore, we presume that the improvement in

the activity of the bimetallic Pd–Pt/ZrAlW catalysts could

be due to a modification of the intrinsic properties of surface

palladium atoms induced by the neighboring platinum

atoms.

Although, the properties responsible for the acidic

character in the palladium catalysts are also observed in

the Pt (0.3 wt.%)/ZrAlW, such as for instance the coex-

istence between W4+ species (BE of 33.42 eV) and W6+

species (BE of 34.93 eV) and an important population % of

pentacoordinated AlV species, however its lower activity

compared with that of the mono- and bimetallic Pd–Pt/

ZrAlW catalysts (Figs. 10 and 11) can be correlated to a

negligible interaction between Pt and WOx species as

inferred from its TPR profile. Another possibility would be

the sintering of platinum upon calcination at 600 8C, since it

is well known that platinum is less resistant to thermal

sintering than palladium [49].

The higher n-hexane conversion of the catalysts with

1.0 wt.% of palladium compared with that of the 0.6 wt.% of

Pd catalysts having the same composition of the mixed oxide

support could be due to a higher density of accessible

metallic sites to dissociate hydrogen required in the

dehydrogenation and rehydrogenation steps in the hydro-

isomerization of n-hexane. The mono- and bimetallic Pd–Pt/

ZrAlW catalysts are complex systems due to many factors

involved in their catalytic performance. In order to elucidate

the role of palladium in the Pd/ZrAlW catalysts as well as

the promotion effect of Pt on the improvement in the

catalytic activity in the hydroisomerization of n-hexane

further experiments must be performed.

5. Conclusions

(1) Addition of Al2O3 to the ZrW mixed oxides prepared by

sol–gel via the hydrolysis assisted by the aqueous

ammonia metatungstate solution generates solids with

higher specific surface area.

(2) Z
irconia in the tetragonal phase predominates in the
mixed oxides and mono- and bimetallic Pd–Pt catalysts,

whereas the addition of Al2O3 to ZrW modifies its

crystallinity and avoids the formation of WO3 bulk

nanocrystals. In these materials, the WOx species are

found highly dispersed.

(3) T
here is a coexistence of various oxidation states of
tungsten on the surface of the mono- and bimetallic Pd–

Pt catalysts after calcination. The presence of reduced-

state WOx species could be ascribed to either an

electronic transfer from the noble metal to the surface

WOx species or the delocalization of the negative charge

onto the WOx network.

(4) T
he structure of the highly dispersed WOx species in
these materials consists of oxotungstate species which

are constituted mainly of monomeric and dimeric

species in tetrahedral coordination.

(5) T
he activity of Pd/ZrW catalysts in the hydroisomeriza-
tion of n-hexane is promoted by both the addition of Al

to the ZrW mixed oxide and the addition of Pt to the Pd/

ZrAlW catalysts.

(6) T
he improvement in the activity of Pd/ZrAlW catalysts
is associated to a lower acid strength and acidity of the

catalyst, which can be related to the coexistence of

surface W6+ and reduced-state WOx species (either W4+

or W0) forming tetrahedrally coordinated oxotungstate

species. The Lewis basic sites of alumina should not be

discarded.

(7) T
he lower acid strength and the amount of acid sites of
the mono-Pd and bimetallic Pd–Pt catalysts supported

on ZrAlW suggests a participation of both Lewis acid

sites (reduced-state WOx species and cus AlV atoms) as

well as Lewis basic sites (cus oxide ions of alumina) on

the skeletal isomerization of n-hexane.

(8) T
he addition of Pt to the Pd/ZrAlW catalyst does not
modify significantly its acidic character. Therefore, the

improvement in the activity in the bimetallic Pd–Pt/

ZrAlW catalysts could be due to a modification of the

intrinsic properties of surface palladium atoms induced

by the neighboring platinum atoms.

Acknowledgements

The authors gratefully acknowledge the technical

assistance of Mr. Carlos Franco in the development of the

RMN experiments. A. Barrera thanks to the Instituto

Mexicano del Petroleo for the scholarship granted during his

post-doctoral stay.

References

[1] A. Corma, J.M. Serra, A. Chica, Catal. Today 81 (2003) 405.

[2] Y. Ono, Catal. Today 81 (2003) 3.

[3] Chong-J. Cao, S. Han, Chang-L. Chen, Nan-P. Xu, Chuny-Y. Mou,

Catal. Commun. 4 (2003) 511.

[4] S.T. Sie, in: G. Ertl, H. Knozinger, J. Weitkamp (Eds.), Handbook of

Heterogeneous Catalysis, vol. 4, Wiley/VCH, Weinheim, 1997, p.

1998.

[5] M. Hino, S. Kobayashi, K. Arata, J. Am. Chem. Soc. 101 (1979) 6439.

[6] J.G. Santiesteban, D.C. Calabro, W.S. Borghard, C.D. Chang, J.C.

Vartuli, Y.P. Tsao, M.A. Natal-Santiago, R. Bastian, J. Catal. 183

(1999) 314.

[7] S.R. Vaudagna, R.A. Comelli, N.S. Fıgoliı, Appl. Catal. 164 (1997) 265.

[8] S.R. Vaudagna, S.A. Canavese, R.A. Comelli, N.S. Figoliı, Appl.

Catal. 168 (1998) 93.

[9] A.V. Ivanov, T.V. Vasina, O.V. Masloboishchikova, E.G. Khelkovs-

kaya-Sergeeva, L.M. Kustov, J.I. Houzvicka, Catal. Today 73 (2002)

95.


[10] E. Iglesia, D.G. Barton, S.L. Soled, S. Miseo, J.E. Baumgartner, W.E.

Gates, G.A. Fuentes, G.D. Meitzner, Stud. Surf. Sci. Catal. 101 (1996)

533.

[11] M. Arribas, F. Marquez, A. Martınez, J. Catal. 190 (2000) 314.

[12] S.-T. Wong, T. Li, S. Cheng, J.-F. Lee, C.-Y. Mou, J. Catal. 215 (2003) 45.

[13] C.-L. Chen, T. Li, S. Cheng, N.-P. Xu, C.-Y. Mou, Catal. Lett. 78

(2002) 223.

[14] W. Hua, Y. Xia, Y. Yue, Z. Gao, J. Catal. 196 (2000) 104.

[15] C. Bigey, L. Hilaire, G. Maire, J. Catal. 198 (2001) 208.

[16] Y. Ono, in: J.M. Thomas, K.I. Zamaraev (Eds.), Perspectives in

Catalysis, Blackwell, London, 1992, p. 533.

[17] D. Barton, Stuart L. Soled, G.D. Meitzner, G.A. Fuentes, E. Iglesia, J.

Catal. 181 (1997) 57.

[18] K. Masuda, M. Kawai, K. Kuno, N. Kachi, F. Mizukami, in: V.G.

Poncelet, P.A. Jacobs, P. Grange (Eds.), Preparation of Catalysts, Stud.

Surf. Sci. and Catal., vol. 63, Elsevier, Amsterdam, 1991, p. 229.

[19] S. Kumar, D.R. Chopra, G.L. Smith, J. Vac. Sci. Technol. B 10 (1992)

1218.

[20] M.A. Arribas, F. Marquez, A. Martınez, J. Catal. 190 (2000) 309.

[21] L. Salvati, L.E. Makovsky, J.M. Stencel, F.R. Brown, D.M. Hercules,

J. Phys. Chem. 85 (1981) 3700.

[22] M. Occhiuzzi, D. Cordischi, D. Gazzoli, M. Valigi, P.C. Heydorn,

Appl. Catal. A: Gen. 269 (2004) 169.

[23] J.R. Regalbuto, T.H. Fleish, E.E. Wolf, J. Catal. 107 (1987) 114.

[24] J.F. Fiedor, A. Proctor, M. Houalla, D.M. Hercules, Surf. Interf. Anal.

23 (1995) 204.

[25] M. Regragui, M. Addou, A. Outzourhit, J.C. Bernede, E.E. Idrissi,

Thin Solid Films 358 (2000) 40.

[26] F. Di Gregorio, V. Keller, J. Catal. 225 (2004) 45.

[27] H.K. Plummer, S. Shinozaki Jr., K.H. Adams, H.S. Gandhi, J. Mol.

Catal. 20 (1983) 251.

[28] W. Grunert, E.S. Shpiro, R. Feldhaus, K. Anders, G.V. Antoshin,

K.H.M. Minachev, J. Catal. 107 (1987) 522.

[29] G.E. McGuire, G.K. Schweitzer, T.A. Carlson, Inorg. Chem. 12 (1973)

2451.

[30] P.G. Gassman, D.W. Maomber, S.M. Willging, J. Am. Chem. Soc. 107

(1985) 2380.

[31] V.M. Benitez, C.A. Querini, N.S. Fıgoli, Appl. Catal. A: Gen. 252

(2003) 427.

[32] I. Wachs, C. Chersich, J. Hardengerg, Appl. Catal. 13 (1985) 335.

[33] V. Benitez, N.S. Fıgoli, Catal. Commun. 3 (2002) 487.

[34] J. Horsley, I. Wachs, J. Brown, G. Via, F. Hardcastle, J. Phys. Chem. 91

(1987) 4014.

[35] K. Fuimoto, F.H. Ribeiro, M. Avalos-Borja, E. Iglesia, J. Catal. 179

(1998) 431.

[36] X.L. Seoane, N.S. Fıgoli, P.C. L’Argentiere, J.A. Gonzalez, A. Arcoya,

Catal. Lett. 47 (1997) 213.

[37] C. Bigey, L. Hilaire, G. Maire, J. Catal. 184 (1999) 406.

[38] M. Sun, T. Burgi, R. Cattaneo, R. Prins, J. Catal. 197 (2001) 172.

[39] R.J. Colton, A.M. Guman, J.W. Rabalais, J. Appl. Phys. 49 (1)

(1978).

[40] P.C. L’Argentiere, N.S. Fıgoli, React. Kinet. Catal. Lett. 64 (2) (1998)

221.

[41] E. Iglesia, Structure and catalytic function of oxide nanostructures, in:

Proceedings of the XIX Simposio Iberoamericano de Catalisis, Mer-

ida, Yucatan, Mexico, September 5–11, 2004.

[42] D.G. Barton, M. Shtein, R.D. Wilson, S.L. Soled, E. Iglesia, J. Phys.

Chem. B 103 (1999) 630.

[43] S. Kuba, P. Lukinskas, R.K. Grasselli, B.C. Gates, H. Knozinger, J.

Catal. 216 (2003) 253.

[44] M.L. Occelli, S. Biz, A. Auroux, P.S. Iyer, Appl. Catal. A: Gen. 179

(1999) 117.

[45] J.A. Wang, X. Bokhimi, A. Morales, O. Novaro, T. Lopez, R. Gomez,

J. Phys. Chem. B 103 (2) (1999) 299.

[46] E. Zhao, S.E. Hardcastle, G. Pacheco, A. Garcia, A.L. Blumenfeld, J.J.

Friapiat, Micropor. Mesopor. Mater. 31 (1999) 9.

[47] C. Bigey, G. Maire, J. Catal. 196 (2000) 224.

[48] H. Knozinger, P. Ratnasamy, Catal. Rev. 17 (1978) 31.

[49] J.C. Summers, D.R. Monroe, Ind. Eng. Chem. Prod. Res. Dev. 20

(1981) 23.

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Isomerization of n-hexane over mono- and bimetallic Pd–Pt catalysts