Al- and Ga-promoted WO3/ZrO2 strong solid acid catalysts and their catalytic activities in n-butane isomerization

Al- and Ga-promoted WO

3

/ZrO

2

strong solid acid catalysts and their

catalytic activities in n-butane isomerization

Xiao-Rong Chen

, Chang-Lin Chen

, Nan-Ping Xu

, Chung-Yuan Mou

b_{Department of Chemistry, National Taiwan University, 1 Roosevelt Road, Section 4, Taipei 106, Taiwan} Available online 3 July 2004

Abstract

A series of Al- and Ga-promoted tungstated zirconia strong solid acid catalysts were prepared. The effect of Al2O3 and Ga2O3on the

structural, acidic and redox features of WO3/ZrO2(WZ) has been characterized by means of X-ray powder diffraction (XRD), N2adsorption

(BET), UV–visible diffuse reflectance spectra (UV–vis), infrared spectra (IR), NH3temperature-programmed desorption (NH3-TPD) and H2

temperature-programmed reduction (H2-TPR). With respect to tungstated zirconia, the promoted catalysts stabilized the zirconia tetragonal

phase, prevented crystalline WO3growth. Al or Ga addition to WZ had less effect on the strength of the acid sites, but effected reduction

temperature of WZ. The isomerization of n-butane was investigated at 300◦C over tungstated zirconia and promoted catalysts. The promoted catalysts improved the isomerization conversion and stability in the presence of hydrogen. The best Al and Ga loading in these catalysts is around 0.5 and 1.0%, respectively, and the optimal calcination temperature is∼850◦C. The experiment results showed that Ga was a more efficient promoter than Al. Pt addition to promoted catalysts showed great improvement on the performance of selectivity and stability. © 2004 Elsevier B.V. All rights reserved.

Keywords: WO3/ZrO2; Al2O3/WO3/ZrO2; Ga2O3/WO3/ZrO2; Strong solid acid; Butane isomerization

1. Introduction

Strong solid acids based on supported metal oxides are potential replacements for liquid acids and halide-containing solid acids with the need for reformulated high-octane gaso-line not containing aromatics and the stringent environment regulation. Among the strong solid acids, sulfated zirconia (SZ) catalysts have attracted significant attention because of their ability to isomerize light alkanes at low tempera-ture [1,2]. Yet, SZ catalysts suffer from the disadvantages of deactivation and possibly from sulfur loss during reaction and regeneration, which limit their applicability in isomer-ization and alkylation processes. Recently, many literatures reported that the stability, activity and selectivity of SZ

cat-alysts could be improved by addition some promoters[3–6].

As an alternative to SZ, tungstated zirconia (WZ) has be-come increasingly important since its discovery by Arata

and Hino [7]. Although WZ catalysts are less active than

SZ, they have superior stability under both reducing and oxidizing conditions and appear to be more suitable for

in-∗_{Corresponding author. Fax:+86 25 83300345.}

E-mail address: [email protected] (C.-L. Chen).

dustrial applications. The catalytic activity of WZ, like that of SZ, can be improved by promotion with platinum and

some metal oxides [8,9]. Moreno and Poncelet [10]found

that the addition of small amounts of Al2O3or Ga2O3to SZ

system enhance the catalytic and the stability for n-butane

isomerization at 250◦C in the presence of H2. The

pro-moting effect of Al or Ga on SZ was later confirmed by

other researchers[6,11]. Al2O3-doped WOx/ZrO2catalysts

and Pt-impregnated forms were claimed as efficient catalysts for the skeletal isomerization of n-butane and other alka-nes[12,13]. Previously, we have reported Ga-promoted WZ greatly improved the catalytic activity for n-butane

isomer-ization[14].

In this paper, n-buatne isomerization has been investigated over Al- and Ga-promoted tungstated zirconias and com-pared with tungstated zirconia. The aim was to investigate the nature of the promoters on the catalytic performances. The influence of platinum addition was also considered.

2. Experimental

Zr(OH)4 was prepared from zirconia nitrate solution

by adding dropwise ammonium hydroxide solution up to 0920-5861/$ – see front matter © 2004 Elsevier B.V. All rights reserved.

pH 9–10 and refluxed for 24 h. The precipitated hydrogel was filtered and washed repeatedly until the filtrate liquid

showed pH 7. The gel was dried at 105◦C. The dried

parti-cles were impregnated with aqueous ammonium tungstate (H8N2O4W, Acoros, 99.999%) in order to obtain W con-tent 10 wt.% in the final catalyst. For all catalysts studied in this work, the W content was 10 wt.%. In the synthesis of Al- or Ga-promoted WZ catalyst, the appropriated amount

of Al(NO)3 or Ga(NO)3 was added to WZ slurry. The

re-sultant suspension was refluxed overnight at 120◦C, dried

at 110◦C, and then calcined at final temperature in static

air for 3 h.

Pt/WZ, Pt/AWZ or Pt/GWZ catalyst was prepared by im-pregnating hydrogen hexachloroplatinate (Acoros, 40% Pt) solution with WZ850, 0.5AWZ850 or 1.0GWZ850 (the for-mer letters mean the promoter weight percent in the final catalyst, the latter letters mean the calcination temperature

in◦C) overnight. The concentration of the solution was

ad-justed in order to obtain 0.3% Pt in the final catalyst. Then it

was dried at 110◦C and calcined at 450◦C in an air stream

for 3 h. The contents of Pt, W, Al, Ga in the final catalysts are the nominal contents.

XRD patterns of the samples were obtained on a Bruker

D8 ADVANCE instrument with Cu K␣ radiation at 40 kV

and 30 mA. BET surface areas of the samples were acquired

on a CHEMBET-3000 instrument using N2 as the

adsor-bent. UV–visible diffuse reflectance spectra (UV–vis) were investigated on a Hitachi U-3010. Infrared spectra (IR) were recorded on a Nicolet 550 spectrometer using a KBr

pel-let. NH3 temperature-programmed desorption (NH3-TPD)

of samples was carried out on a Micromeritics AutoChem 2910 instrument. 0.2 g calcined sample was used for each

ex-periment. Prior to NH3adsorption, sample was pretreated at

450◦C in flowing air for 1 h in order to clean the surface from

adsorbed species, and then cooled down in He. The NH3

ad-sorption was carried out at 100◦C. After the saturation

ad-sorption of NH3, the carrier gas He was allowed to flow over

the sample at 100◦C for 0.5 h. The desorption of NH3was

started at 100◦C and continued until 800◦C at 10◦C/min.

The desorption process was monitored by a Quadruple Mass Spectrometer (Thermo ONIX, ProLab) connected on line through a heated capillary interface. The mass number 16

was followed to obtain TPD profiles of NH3 because the

mass intensity is relatively strong and the interference from

H2O is negligible. H2 temperature-programmed reduction

(H2-TPR) of samples was performed on a CHEMBET-3000

instrument equipped with a thermal conductivity detector (TCD). About 0.2 g sample was pretreated in a flowing air

at 400◦C for 1 h, cooled to room temperature in N2, and

then heated to∼800◦C at a rate of 10◦C/min in 20 ml gas

stream of 10% H2in Ar.

The isomerization of n-butane was performed in a fixed-bed flow reactor. Prior to reaction, the catalyst was

pretreated at 450◦C for 3 h under air condition, and then

contacted with flowing hydrogen at reaction temperature for 1 h. The following reaction condition was used: reaction

temperature, 300◦C; pressure, 101.3 kPa; catalyst mass,

1.0 g; feed flow rate (at NTP), 3 ml/min of n-butane mixed

with 12 ml/min of H2. An on-line GC-14C gas

chromato-graph equipped with FID was used to analyze the reaction products.

3. Results and discussion 3.1. BET surface area

The BET surface areas of the AWZ and GWZ samples with different Al, Ga content and calcinations temperature

were measured, and the data are presented inTable 1. The

addition of Al or Ga does not change the specific surface

ar-eas of WZ samples. Samples calcined at 850◦C have nearly

same surface areas about 55 m2/g, and the higher the

calci-nations temperature, the smaller the surface areas. 3.2. X-ray diffraction

Fig. 1 shows the XRD patterns of samples calcined at different temperature. All samples calcined at 800 and

850◦C show peaks characteristic of the tetragonal phase

zirconia, the main peak appearing at 2θ = 30.2◦ and no

WO3crystallite was observed. As temperature increased to

900◦C, WZ900 presents a mixture of a monoclinic phase

and a tetragonal phase of zirconia, 0.5AWZ900 shows two

small peaks of monoclinic zirconia (2θ = 28.5◦, 31.5◦), but

no monoclinic phase is observed from 1.0GWZ900. The

diffraction peak of WO3 crystallite (2θ = 23–25◦) on the

zirconia surface can be detected with an increase of the

calcination temperature to 900◦C. The intensity of

crys-talline WO3 in 0.5AWZ900 and 1.0GWZ900 was weaker

than that in WZ900. The stabilization of tetragonal zirconia by various metal-oxide dopants and sulfate is well-known

phenomenon in the materials fields[11,13,15–17]. It can be

explained that aluminum and gallium retard the growth of

crystalline WO3on the surface of zirconia and suppress the

monoclinic zirconia. As the specific surface area dropped

and the crystallinity of ZrO2 increased during calcinations

Table 1

Surface area of various catalysts

Samples Promoter content (%) Surface area (m2/g)

WZ850 0 53.3 AWZ800 0.5 72.0 AWZ850 0.3 51.2 AWZ850 0.5 52.0 AWZ850 0.7 55.0 AWZ900 0.5 31.8 GWZ800 1.0 77.6 GWZ850 0.5 53.3 GWZ850 1.0 58.1 GWZ850 1.5 57.6 GWZ900 1.0 31.0

10 20 30 40 50 60 70 (3) (2) (1) 800oC A (3) (2) (1) 850oC W T M M (3) (2) (1)

Intensity (a.u.)

2 Theta

Fig. 1. XRD patterns of samples: (1) WZ, (2) 0.5AWZ and (3) 1.0GWZ. T—tetragonal zirconia; M—monoclinic zirconia; W—crystalline WO3.

at high temperature, these resulted in the formation of

mi-crocrystalline WO3observed. It was known[18,19]that the

tetragonal structure was essential in highly active acid cata-lysts and monoclinic phase was poorly efficient in n-butane isomerization. This suggested that aluminum and gallium influence catalytic activity through the crystallization be-havior. It favored the formation of the tetragonal phase and stabilized tetragonal zirconia crystallites even at higher calcination temperature.

3.3. UV–visible diffuse reflectance spectra

UV–visible diffuse reflectance spectroscopy was used to probe tungsten oxide species dispersed on zirconia surface

(showed inFig. 2). The size effect can be reflected from the

shift of the absorption edge[20]. Because pure WO3has a

large size of crystallization, it gave the highest absorption edge near 450 nm that is the characteristic edge of crystalline

WO3among the samples examined. To WZ850, with WOx

well dispersed on zirconia surface, one obtained small size

of crystalline WO3, so WZ850 showed weak absorption

in-tensity near 450 nm. The size of crystalline WO3 became

smaller after Al or Ga introduced to WZ. No absorption edge

of crystalline WO3 on 0.5AWZ850 and 1.0GWZ850 near

450 nm was detected. It indicated that Al and Ga efficiently

suppressed the growing of crystalline WO3. The results are

qualitatively in accord with the conclusions obtained from X-ray diffraction. 200 300 400 500 600 700 800

Absorbance

Wavelength (nm)

Fig. 2. UV–visible diffuse reflectance spectra of (1) WO3, (2) WZ850, (3) 0.5AWZ850 and (4) 1.0GWZ850.

3.4. Infrared spectra

Fig. 3 illustrated the IR spectra of WZ850, 0.5AWZ850

and 1.0GWZ850. The band at 750 cm−1is the

characteris-tic of monoclinic zirconia. For Al- and Ga-promoted

sam-ples, the intensity of the band at 750 cm−1was weaker than

WZ850. 1.0GWZ850 sample exhibited the weakest

inten-sity at 750 cm−1among the three samples. This further

con-firms that the monoclinic phase was restrained when Al or Ga introduced to WZ. Promotion by Al or Ga enhanced the stability of the tetragonal structure with respect to tungstated zirconia and the fractions of tetragonal structure are found to play an essential role in the catalytic activity. It was also confirmed that Ga was a more efficient promoter than Al. 3.5. Acidity measurements

We used NH3-TPD technique to compare the acidic

characteristic of the same set of WZ850, 0.5AWZ850 and

500 1000 1500 2000

Absorbance

Wavenumber (cm

)

Fig. 3. Infrared spectra of (1) WZ850, (2) 0.5AWZ850 and (3) 1.0GWZ850.

100 200 300 400 500 600 700 800 (3) (2) (1)

NH

+

Intensity (a. u.)

Temperature (

_C)

Fig. 4. NH3-TPD profiles of (1) WZ850, (2) 0.5AWZ850 and (3) 1.0GWZ850.

1.0GWZ850 catalysts.Fig. 4shows that there is no obvious

difference in the strength of the acid sites among these three samples. This suggested that the promoter of Al or Ga does not affect the acidity of WZ.

3.6. H2-TPR of WZ, AWZ and GWZ

To understand the promotion effect of Al and Ga, we

used H2temperature-programmed reduction (H2-TPR)

tech-nique to compare the characteristic of WZ850, 0.5AWZ850 and 1.0GWZ850. It has been suggested that reduction of

tungstated zirconia with H2at elevated temperatures leads to

the formation of new OH groups associated with W5+ and

that those OH groups or the W5+ sites may play a crucial

role in the mechanism of the isomerization of small alkanes [21–23]. According to the literature[24], pure WO3exhibits

three reduction peaks, namely a shoulder 540◦C (WO3→

W20O58), a sharp peak at 775◦C (W20O58 → WO2) and a

peak at higher temperature (WO2→ W). Pure ZrO2 does

not show any detectable TPR peak at temperature below

1000◦C. So the reduction peaks observed from Fig. 5 can

be attributed to the reduction of WO_x. The H2-TPR

pro-file of WZ850 presents a peak of hydrogen consumption at

450◦C. 0.5AWZ850 shows two peaks at 450 and 650◦C,

respectively, and 1.0GWZ850 shows a peak at 375◦C. The

profiles indicate that the first reduction temperature of WZ and AWZ was the same. The second reduction temperature of WZ was not detected until the reduction temperature

in-creased to∼800◦C, but it was decreased after addition Al

to WZ. Addition of Ga to WZ decreased the first reduc-tion temperature of WZ and undetected the second reducreduc-tion temperature in the testing range. It was suggested that Al or

Ga addition improved the redox properties of W6+ and Ga

is a more efficient promoter to the redox properties of W6+.

The redox properties of WZ had been observed and related

to the activation of n-alkane[25,26]. Recently, many

liter-atures [25–27] reported that the redox properties of W6+

200 300 400 500 600 700 800 (3) (2) (1) TCD Signal (a.u.)

Temperature (

C)

Fig. 5. H2-TPR profiles of (1) WZ850, (2) 0.5AWZ850 and (3) 1.0GWZ850.

in WO_x/ZrO2 played a crucial role as a redox initiator in

activation of alkanes. In our study, the catalysts show the catalytic activity in the order 1.0GWZ850 > 0.5AWZ850 >

WZ850 in correspondence with the H2-TPR results. We can

infer that 1.0AWZ850 and 1.0GWZ850 might be bifunc-tional, exhibiting both acidic and redox properties, and that alkane isomerization might be initiated by oxidation of the alkane. The decreased reduction temperature of 0.5AWZ850 and 1.0GWZ850 may make n-butane isomerization easier. 3.7. Catalytic activity in n-butane isomerization

The dependence of initial catalytic activity of promoted

catalysts calcined at 850◦C on promoter content is shown

in Fig. 6. The conversion of n-butane at 10 min is as the initial activity. All promoted catalysts exhibited higher ini-tial conversion than WZ850. For Ga-promoted catalysts, the n-butane conversion increased with the Ga content up to

0.0 0.5 1.0 1.5 0 10 20 30 40 n-C 4 Conversi on (% ) Ga Content (%) 0.0 0.2 0.4 0.6 0.8 0 5 10 15 20 25 Al Content (%) Fig. 6. The effect of promoter content in WZ catalysts on n-butane isomerization. All samples are calcined at 850◦C. Reaction temperature: 300◦C; WHSV: 0.47 h−1; H2/n-C4 = 4.

Table 2

Calcination temperature effect on initial activity

Samples 1.0AWZ 0.5GWZ

Calcination temperature(◦C) 800 850 900 800 850 900

Initial conversion (%) 16.4 22.6 12.9 26.2 36.0 29.4

1.0 wt.%, and then decreased, as the Ga content was further increased. We also obtained the optimal Al loading in WZ is 0.5 wt.%. Compared to the low initial activity of WZ850 about 5.7%, the initial conversion of n-butane over WZ850 with the best Ga and Al loading reaches 37.3 and 22.6%, re-spectively. It was observed that gallium is a better promoter than aluminum under comparable condition.

Table 2shows the dependence of n-butane conversion on calcination temperature. We investigated the catalytic ac-tivity of 0.5AWZ and 1.0GWZ samples over the range of

800–900◦C. It was found that calcination at 850◦C for both

0.5AWZ and 1.0GWZ was more effective than at 800 or

900◦C. A possible explanation is that tungstated zirconia

catalysts must be calcined at high temperature to develop the

catalytic activity toward alkanes isomerization [28].

How-ever, high calcination temperature favors monoclinic zirco-nia formation, which would result in a material of distinctly lower acidity. Furthermore, the higher the calcination tem-perature, the lower the surface area one obtains, which

re-sults in lower catalytic activity. So calcination at 850◦C

seems to be an optimum temperature for tungstated zirconia catalysts.

The conversions and selectivity to i-C4versus time curves

obtained at 300◦C over the Pt-impregnated and Pt-free

catalysts are compared in Fig. 7. Promotion with Al or

Ga greatly enhanced the catalytic properties and stability relative to pure tungstated zirconia. Although 1.0GWZ850 showed higher n-butane conversion than 0.5AWZ850, the

stable selectivity to i-C4 of 1.0GWZ850 was ∼85%, the

same as 0.5AWZ850. Pt impregnation further increased the catalytic performances with respect to the Pt-free forms.

0 60 120 180 240 300 0 10 20 30 40 50 0 60 120 180 240 30030 40 50 60 70 80 90 100

n-C

C

o

nve

rsion (%

)

Time on Stream (min)

Selec

tivity

to i-C

4

_(%

)

Fig. 7. The catalytic performances vs. time-on-stream over the cata-lysts. WZ850 (䊏), 0.5AWZ850 (䊉), 1.0GWZ850 (䉱), Pt/WZ850 (䉲), Pt/0.5AWZ850 (䉬), Pt/1.0GWZ850 (
).

The addition of Pt to WZ850 does improve in activity of

the catalyst for n-butane isomerization at 300◦C, but does

not improve the selectivity to i-C4. The selectivity to i-C4

over Pt/WZ850 was∼40%, much lower than WZ850, in

ac-cordance with previous literatures [29,30]. Pt/1.0GWZ850

immediately attained a stable conversion of 42% and se-lectivity of 94%. The initial activity of Pt/0.5AWZ850 was very high, reaching 50%, but the initial selectivity was much low because the cracking productions were rich. With time-on-stream, the cracking productions decreased. Pt/0.5AWZ850 attained a stable conversion of 40% and selectivity of 91.5%. At steady conversion, Pt/1.0GWZ850 was more active than Pt/0.5AWZ850.

4. Conclusions

Promotion of tungstated zirconia with Al and Ga im-proved the efficiency and stability of tungstated zirconia in n-butane isomerization. The optimal promoter loading in

AWZ or GWZ calcination at 850◦C is 0.5 or 1.0%,

respec-tively. From the results obtained from this study, Ga was a more efficient promoter than Al. Platinum addition sig-nificantly increased the overall n-butane conversion and the selectivity to i-butane of all promoted catalysts. These cata-lysts show the catalytic activity in the order Pt/1.0GWZ850 > Pt/0.5AWZ850 > 1.0GWZ850 > 0.5AWZ850 > WZ850 under the identical reaction condition.

In contrast to WZ, Al- and Ga-promoted WZ showed less difference in acidity characteristic, but had lower reduction

temperature, which improved the redox properties of W6+.

The promoting effect of Al or Ga is suggested to be a combi-nation of several possible factors, such as (1) enhancing the stability of the tetragonal structure; (2) affecting the

crystal-lite size of WO3on the surface of WZ; (3) improving the

redox properties of W6+.

Acknowledgements

We acknowledge the financial supports from the Science and Technology Department of Jiangsu Province (Project BG2002017), and the Key Laboratory of Material-Oriented Chemical Engineering of Jiangsu Province.

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