Catalytic combustion of n-pentane on Pt supported on solid superacids

Weiming Hua, Zi Gao*

Department of Chemistry, Fudan University, Shanghai 200433, PR China

Received 3 October 1997; received in revised form 17 November 1997; accepted 18 November 1997

Abstract

The combustion of n-pentane on Pt supported on sulfated zirconia-based solid superacids has been studied. The combustion

activity is related with the Pt dispersion of the catalyst and the acid strength of the support. Chemisorption of n-pentane on the

catalysts has been investigated by means of infrared spectroscopy. The formation of alkane carbocations on the surface

sulfates during reaction may account for the enhancement of the combustion activity by the strong acidity of the supports.

# 1998 Elsevier Science B.V.

Keywords: n-Pentane catalytic combustion; Pt catalyst; Superacid support; Chemisorption

1. Introduction

The discovery of sulfated zirconia-based solid

superacid catalysts has opened new perspectives in

the use of environment friendly solid catalysts for

reactions involving very strong acid sites. Numerous

studies have been devoted to developing new pro-

cesses based on solid superacid catalysts to replace

HF, H2SO4 and AlCl3, such as alkylation of isobutane

with butenes, isomerization of butane and some cat-

alytic acylation reactions [1,2]. Supported Pt catalysts

are the most active catalysts for low-temperature

catalytic combustion. Recently, it was found that

the catalytic activities of Pt=SO2ÿ4 =ZrO2 for the com-

bustion of various organic compounds are much

higher than those of Pt/Al2O3 [3]. In the combustion

of alkanes, such as hexane and heptane, the difference

in the complete combustion temperature of these two

types of catalysts reaches 1008C. Preliminary experi-

mental results show that superacidity of the sulfated

zirconia supports is primarily responsible for the

improvement in activity rather than other factors.

The aim of the present work was to study factors

affecting the activity of Pt supported on sulfated

zirconia-based materials for combustion of n-pentane

as a representative of alkanes in more detail, and to

elucidate the reason for the promoting effect of the

superacidic support on alkane combustion reaction.

2. Experimental

Amorphous Zr(OH)4 precipitate prepared from

ZrOCl2�8H2O was immersed in a 0.5 M H2SO4 solu-

tion for 30 min, and then followed by ®ltration, dying

and calcination at 500±8008C for 3 h. The sulfated

zirconia samples were labeled as SZ. Mixed oxides

containing Zr and other metals, such as Cr, Fe, Mn, Bi

and V, were prepared by co-precipitation or impreg-

Applied Catalysis B: Environmental 17 (1998) 37±42

*Corresponding author. Tel.: +86 21 65642792; fax: +81 21

65641740; e-mail: zigao@fudan.ac.cn

0926-3373/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.

P I I S 0 9 2 6 - 3 3 7 3 ( 9 7 ) 0 0 1 0 0 - 8

nation method, and then they were immersed in a

0.5 M (NH4)2SO4 solution, ®ltered, dried and calcined

at 6508C for 3 h. The sulfated mixed oxides containing

1.5 wt% Cr, 1.5 wt% Fe�0.5 wt% Mn, 1.5 wt%

Fe�0.5 wt% Bi and 1.5 wt% Fe�0.5 wt% V were

labeled as SCZ, SFMZ, SFBZ and SFVZ, respectively.

The supported Pt (0.5 wt%) catalysts were prepared by

impregnating the sulfated oxides with an aqueous

solution of H2PtCl6, drying at 1108C and calcining

in dry air at 4008C for 3 h.

BET surface area of the catalysts was measured on a

Micromeritics ASAP 2000 system under liquid N2

temperature using N2 as the adsorbate. The dispersion

of Pt was detected by CO pulse-adsorption method. Pt

catalysts were prereduced in situ in ¯owing H2 at

3508C for 3 h before measurement. Pt dispersion was

calculated from the total CO uptake by assuming a

stoichiometry of [CO]/Pts�1.

XPS spectra were recorded on a Perkin-Elmer PHI-

5000C ESCA system with Al K� radiation. A binding

energy of 182.7 eV for the Zr (3d5/2) level was used as

an internal reference for all the samples. IR spectra

were recorded on a Perkin-Elmer 983G spectrometer.

The samples were pressed into self-supported disks

with a density of 3±5 mg/cm2, and placed in a quartz

cell with CaF2 windows.

Chemical method was used for the detection of

sulfate content in the samples. Dehydrated Na2CO3

and ZnO (1:4) were used as the fusing agents, and the

sulfate in the samples was turned into BaSO4 and

determined by gravimetric method. Superacidity of

the samples was measured by a modi®ed Hammett

indicator method [4] and n-butane isomerization test

reaction [5].

Combustion reaction tests were carried out on a

continuous ¯ow reaction apparatus. 0.67 ml of catalyst

(40±60 mesh) was mixed with quartz powder in the

ratio of 1:5 to prevent undesired temperature rise in the

reactor. The catalyst was pretreated in a ¯ow of H2 at

3508C for 3 h, and then in a ¯ow of air at 5508C for

2 h. The reactant was a gas mixture of air and n-

pentane with a concentration of 29.5 g/m3, and the

space velocity (GHSV) was 5000 hÿ1. Reaction tem-

perature was measured by a thermocouple installed at

the center of the catalyst bed. After the bed tempera-

ture was steadied for 30 min, the reaction products

were collected and analyzed by using a gas chroma-

tograph equipped with a FID detector.

3. Results and discussion

3.1. Activity of Pt catalysts supported on various

superacids

The catalysts used in the present work and their

important physicochemical properties are given in

Tables 1 and 2. The isomerization of n-butane on

solid superacids at 358C obeys the rate law of ®rst-

order reversible reaction, and the forward rate constant

k1 can be considered as a measure of superacidity of

the SZ samples [5].

Fig. 1 shows the n-pentane combustion activities of

Pt/SZ series catalysts as a function of reaction tem-

perature. The combustion activity of the catalysts

depends strongly on the calcination temperature of

the SZ support, and the activity of all the catalysts

except for Pt/SZ(800) increases rapidly with reaction

Table 1

Properties and combustion activities of Pt/SZ catalysts

Catalysta SO3 (wt%) Support acid strength Surface area (m2/g) Pt dispersion (%) T90 (8C)

Ho n-C4 isomerizationb (k1�103/hÿ1)

Pt/SZ(500) 7.9 ÿ12.7 16.4 148 19 210

Pt/SZ(600) 4.6 ÿ14.5 21.7 124 35 230

Pt/SZ(650) 3.3 ÿ16.0 40.1 107 36 225

Pt/SZ(700) 1.5 ÿ13.8 21.0 51 31 270

Pt/SZ(800) 0.8 ÿ12.7 8.98 7 8 280

Pt/Al2O3 Ð �4.8 0 314 63 320

aFigures in parenthesis are the calcination temperatures (8C) of the superacid support.bReaction temperature is 358C.

38 W. Hua, Z. Gao / Applied Catalysis B: Environmental 17 (1998) 37±42

temperature and exhibits a typical sigmoid shape

curve. The temperatures of 90% conversion were read

out from the curves and listed in Table 1 as a measure

of the combustion activities. It is evident that combus-

tion activities of Pt/SZ catalysts are higher than that of

Pt/Al2O3 under the same reaction conditions. Fig. 2

depicts the change of the properties of SZ supports and

Pt catalysts and their combustion activities with the

calcination temperature of the supports. As the calci-

nation temperature is raised, the acid strength (Ho and

k1) of the SZ supports goes through a maximum at

6508C, whereas the surface area and SO3 content of

the catalysts decrease with the calcination tempera-

ture. The Pt dispersions of Pt/SZ(600), Pt/SZ(650) and

Pt/SZ(700) are similar, but they are much higher than

those of the other two catalysts. The low Pt dispersions

of Pt/SZ(500) and Pt/SZ(800) are probably caused by

the high sulfate concentration and low surface area of

the supports, respectively. For the three Pt catalysts

with identical metal dispersion, their combustion

activities are in the order of Pt/SZ(650)>Pt/

SZ(600)>Pt/SZ(700), showing that the activity

changes concurrently with the acid strength of the

supports. The high combustion activity of Pt/SZ(500)

is probably due to its high SO3 content, which means

that there may be more strong acidic sites in the

catalyst appropriate for the reaction although their

acid strength is lower than that of SZ(650).

Recently, Hsu et al. [6] reported that sulfated trinary

oxides containing Fe, Mn and Zr exhibited a higher

activity for n-butane isomerization than SZ. At the

same time, a wide variety of sulfated binary and

trinary oxides were prepared and characterized in

our laboratory. It was found that sulfated oxides of

Cr±Zr, Fe±Cr±Zr and Fe±V±Zr were 2±3 times more

active for n-butane isomerization than sulfated

Table 2

Properties and combustion activities of Pt supported on metal promoted superacids

Catalysta SO3 (wt%) Support acid strength Surface area (m2/g) Pt dispersion (%) T90 (8C)

n-C4 isomerizationb (k1�103/hÿ1)

Pt/SFBZ(650) 4.4 95.2 104 9 275

Pt/SFMZ(650) 4.2 134.8 97 12 280

Pt/SCZ(650) 3.8 290.6 118 24 255

Pt/SFVZ(650) 4.1 350.5 86 10 285

aFigures in parenthesis are the calcination temperatures (8C) of the superacid support.bReaction temperature is 358C.

Fig. 1. Combustion activities of Pt/SZ series catalysts: (&) 5008C;

(~) 6008C; (*) 6508C; (&) 7008C; (!) 8008C.

Fig. 2. Relationship between pentane combustion activities of

Pt/SZ series catalysts and their properties.

W. Hua, Z. Gao / Applied Catalysis B: Environmental 17 (1998) 37±42 39

Fe±Mn±Zr oxide [7,8], as shown in Table 2. Com-

bustion activities of Pt supported on some of these

new superacids were tested and given in Table 2. The

combustion activities of the catalysts are in the order

of Pt/SCZ>Pt/SFBZ�Pt/SFMZ>Pt/SFVZ. These

catalysts are more active than a conventional

Pt/Al2O3 catalyst but less active than a Pt/SZ(650)

catalyst, implying that the new superacids do not

possess extremely high superacidity as expected. This

is consistent with the suggestion that the promoting

effect of the transition metals in n-butane isomeriza-

tion reaction is associated with an enhanced surface

concentration of C4 ole®n due to their dehydrogena-

tion activity rather than an increase in acid strength,

since n-butane isomerization at low temperature pro-

ceeds via a bimolecular mechanism involving C8

intermediates [2,9]. On the other hand, the lowering

of the Pt dispersion in these catalysts is probably

responsible for the reduction in activity.

The above experimental results demonstrate that

both the acid strength of the support and the Pt

dispersion of the catalyst are important for low-tem-

perature catalytic combustion of alkanes. Pt/SZ cata-

lysts with high superacidity and moderate Pt

dispersion are more appropriate for the reaction than

Pt/Al2O3 catalyst with low acidity and high Pt dis-

persion, hence the largest difference in T90 between

these two types of catalysts amounts to 1108C.

3.2. XPS study of the state of Pt and other metals

The combustion activity of Pt catalysts depends on

the state of platinum. It has been reported that metallic

Pt is more active than oxidized Pt [10]. The state of Pt

in Pt/SZ catalysts is a matter of controversy. Ebitani et

al. [11] reported that in the presence of sulfate ions Pt

remains essentially in an oxidized state even after

hydrogen reduction at 4008C. However, Sayari et al.

[12,13] pointed out that Pt is reduced to the metallic

state even after air calcination of the sulfated sample at

6008C. XPS spectra of the catalysts used in the present

work were recorded after in situ reduction and oxida-

tion treatments. The Pt(4f) XPS spectra of Pt/SZ(650)

were shown in Fig. 3. The Pt(4f7/2) binding energy and

the binding energies of all the other metals in the

catalysts were listed in Table 3.

After reduction in ¯owing H2 at 3508C, platinum in

the catalysts is essentially in the metallic state with a

binding energy of 71.4±71.8 eV. The binding energy

Fig. 3. Pt(4f) XPS spectra of Pt/SZ(650) catalyst: (a) pretreated in

a flow of H2 at 3508C for 3 h; (b) followed by air oxidation at

5508C for 2 h.

Table 3

Binding energies of various elements on catalyst surface

Catalyst Binding energy (eV)

Pt(4f7/2) Cr(2p3/2) Fe(2p3/2) Mn(2p3/2) V(2p3/2)

Aa Bb A B A B A B A B

Pt/SZ(650) 71.4 73.0 Ð Ð Ð Ð Ð Ð Ð Ð

Pt/SCZ(650) 71.7 73.2 577.0 577.3 Ð Ð Ð Ð Ð Ð

Pt/SFMZ(650) 71.7 73.2 Ð Ð 711.4 711.8 642.3 642.5 Ð Ð

Pt/SFVZ(650) 71.8 73.3 Ð Ð 711.9 711.9 Ð Ð 520.7 520.8

aPretreated in a flow of H2 at 3508C for 3 h.bPretreated in a flow of H2 at 3508C for 3 h, and then in a flow of air at 5508C for 2 h.

40 W. Hua, Z. Gao / Applied Catalysis B: Environmental 17 (1998) 37±42

of Pt(4f7/2) shifts to 73.0±73.3 eV after reoxidation in

¯owing air at 5508C, indicating that Pt has been

oxidized to PtO2. Therefore, the role of the sulfated

oxide supports in the enhancement of the combustion

activity cannot be ascribed to the prevention of the

oxidation of supported platinum as described by Ishi-

kawa et al. [14].

The binding energies of Cr(2p3/2), Fe(2p3/2),

Mn(2p3/2) and V(2p3/2) are essentially unaltered after

reduction and oxidation treatments, showing that the

transition metal promoters in the catalysts remain in

the state of Cr3�, Fe3�, Mn4� and V5�. This result

excludes the assumption that the reduction in activity

for Pt supported on sulfated binary and trinary oxides

in comparison with Pt/SZ is related to variations in the

properties of the supports caused by changes in

valence states of the promoters.

3.3. Chemisorption and combustion reaction

Chemisorption of n-pentane on the Pt catalysts was

studied by IR spectroscopy. Spectrum taken after

adsorption of C5H12 on Al2O3 and subsequently

degassed at ambient temperature shows no evidence

of chemisorption. On Pt/Al2O3 chemisorption of

C5H12 is observed as shown in Fig. 4. The bands at

2958, 2924 and 2863 cmÿ1 are the stretching vibration

bands of C±H bonds. This shows that Pt species on

Al2O3 promotes the chemisorption of C5H12. Unlike

the spectrum of Al2O3, IR spectra of SZ(650) and

SCZ(650) taken after adsorption of C5H12 display

C±H vibration bands, implying that C5H12 can be

chemisorbed on these supports and form adsorbed

carbenium ions. The three C±H vibration bands are

further intensi®ed on the spectra of Pt/SZ(650) and Pt/

SCZ(650) catalysts. Similar to our results, the bands at

2960, 2920 and 2880 cmÿ1 were observed on the

spectrum of sulfated Pt/Al2O3 after adsorption of

propane [15], and the bands at 2957 and 2926 cmÿ1

were observed on the spectrum of SO2ÿ4 /Fe2O3 after

adsorption of n-butane [16].

Fig. 5 depicts the IR spectra in the range of 1700±

1100 cmÿ1 of the samples. A strong adsorption band

corresponding to the asymmetric stretching frequency

of O=S=O is observed for all the samples. The S=O

band of SZ(650) and Pt/SZ(650) is located at

1385 cmÿ1 and it shifts to 1333 cmÿ1 after C5H12

adsorption, indicating a strong interaction between the

C5H12 molecules and the surface sulfates of the sup-

port. A similar shift of the S=O band from 1370 to

1327 cmÿ1 is found on the spectra of SCZ(650) and

Fig. 4. IR spectra (3200±2600 cmÿ1) of n-pentane adsorption on

(a) Al2O3; (b) SZ; (c) Pt/SZ(650); (d) SCZ(650); (e) Pt/SCZ(650);

(f) Pt/Al2O3.

Fig. 5. IR spectra (1700±1100 cmÿ1) of (a) SZ(650); (b) Pt/

SZ(650); (c) SCZ(650); (d) Pt/SCZ(650) and after n-pentane

adsorption on (e) SZ(650); (f) Pt/SZ(650); (g) SCZ(650); (h) Pt/

SCZ(650).

W. Hua, Z. Gao / Applied Catalysis B: Environmental 17 (1998) 37±42 41

Pt/SCZ(650) after C5H12 adsorption. The same band

shifts in the sulfate region have been reported for SZ

and analogous catalysts after adsorption of pyridine

[17] and water [7]. The interaction between the adsor-

bate and surface sulfate complex has been elucidated

as that of the adsorption of pyridine or water on the

strong Lewis acid site, namely the central metal

cation, reduces the double bond character of S=O

[17]. Our experimental results show that pentane like

pyridine and water can also interact with the strong

Lewis acid site and generate adsorbed carbenium ion

via hydride abstraction.

Dioxygen chemisorbs dissociatively on Pt catalyst

at a rather high rate. The extent of surface coverage

with oxygen reported in the literature is 0.5 [18] or

close to unity [19] when oxygen is present in the

reaction mixture. The complete oxidation reaction is

initiated with dissociation chemisorption of the alkane

during which the weakest C±H bond of the alkane is

broken, and the reaction will be most favorable when

the catalyst surface has the ability to maintain frac-

tional surface coverage with both hydrocarbon and

oxygen [18].

The observed differences between the catalytic

properties of Pt/Al2O3 and Pt/SZ and its analogs are

likely associated with variations in their ability to

activate n-alkane molecules. On Pt during chemisorp-

tion the formation of an alkyl radical with the loss of a

single hydrogen occurs

However, on SZ and its analogs adsorbed carbe-

nium ion is formed as a result of the interaction of the

alkane with the strong Lewis acid site:

n-C5H12 � L! sec-C5H�11 � LHÿ:

Since the coexistence of chemisorbed hydrocarbon

and oxygen on neighboring sites on the catalysts is an

essential requirement for complete oxidation, the

additional activation of the alkane molecules by the

strong Lewis acid sites on the supports leads to a

higher surface coverage of the alkane and in result

promotes the reaction. Hence, the combustion activity

of the Pt/SZ catalysts with almost identical Pt disper-

sion increases with the acid strength of the superacidic

supports as seen in Table 1.

From the results of our measurements and informa-

tion in the literature, it can be concluded that both Pt

dispersion and acid strength of the support are impor-

tant for alkane combustion catalysts. An appropriate

match of these two properties in a catalyst is favorable

for complete oxidation, because in such a case the

catalyst surface maintains adequate fractional surface

coverage with both reactants, namely, the activated

alkane and dissociated oxygen. Using sulfated oxides

as supports for combustion catalysts provides more

chances to obtain active catalysts due to their high acid

strength and unique ability to activate n-alkanes.

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