regenerar tungphosporic

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Applied Catalysis A: General 214 (2001) 47–58

Coking and regeneration of H3PW12O40 /SiO2 catalysts

Ivan V. Kozhevnikov∗, Stephen Holmes, M.R.H. Siddiqui Department of Chemistry, Leverhulme C entre for Innovative Catalysis, University of Liverpool, Liverpool L69 3BX, UK

Received 27 August 2000; received in revised form 20 December 2000; accepted 21 December 2000

Abstract

The coking during propene oligomerisation over silica-supported heteropoly acid (HPA) H3PW12O40 (PW) and its

palladium-doped form (1.6–2.5 wt.% Pd) and subsequent catalyst regeneration have been studied. Coke formation has been

found to cause rapid deactivation of the catalysts. The coked versus fresh catalysts have been characterised by 31 P and 13C

MAS NMR, XRD, XPS and TGA/TPO to reveal that the Keggin structure of the catalysts was unaffected by coke depositionin both undoped and Pd-doped PW/SiO2. The Pd doping has been shown to affect the nature of coke formed, inhibiting the

formation of polynuclear aromatics. Addition of water, methanol or acetic acid to the propene flow causes the formation

of oxygenated products at the expense of propene oligomers. These additives have been found to inhibit the coking, water

being the most effective inhibitor. The removal of coke from HPA catalysts has been attempted using solvent extraction,

ozone treatment and aerobic oxidation. The extraction (e.g. with CH2Cl2) allows removing soft coke (with the TGA removal

range of 170–370◦C) but is unable to remove hard coke (with the TGA removal range of 370–570◦C). Ozone treatment can

remove both soft and hard coke at 150◦C. The aerobic burning of coke on the undoped PW/SiO 2 proceeds to completion

in the temperature range centred at 500–560◦C, exceeding the temperature of PW decomposition. Doping the catalyst with

Pd significantly decreases this temperature to allow catalyst regeneration at temperatures as low as 350◦C without loss of

catalytic activity. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Heteropoly acid; Palladium doping; Propene oligomerisation; Coke formation; Catalyst regeneration

1. Introduction

Heteropoly acids (HPAs) have been extensively

studied as acid and oxidation catalysts for many reac-

tions and found industrial application in several pro-

cesses [1–6]. HPAs are promising solid acids to replace

environmentally harmful liquid acid catalysts such as

H2SO4 [1–4]. The HPA-based solid acid catalysts,

especially those comprising the strongest Keggin-

type HPA such as H3PW12O40 (PW) or H4SiW12O40

(SiW), are more active than conventional solid acids

∗ Corresponding author. Tel.: +44-151-794-2938;

fax: +44-151-794-3589.

E-mail address: [email protected] (I.V. Kozhevnikov).

such as SiO2-Al2O3, H3PO4 /SiO2 and zeolites [1,4].

Their use, however, is limited because of the difficulty

of HPA regeneration [1]. Generally, in acid-catalysed

organic conversions, solid acid catalysts are deacti-

vated by coke formation. In the case of conventional

catalysts such as SiO2-Al2O3 or zeolites, regeneration

can be successfully achieved by a controlled burning

of the deposited coke with oxygen at 450–550◦C

[7,8]. In the case of HPA catalysts, this method is not

applicable as they have insufficient thermal stability.

The most commonly used HPAs, PW and SiW, decom-

pose above 465 and 445◦C, respectively [9]. Given

the relatively low thermostability of HPAs, the devel-opment of a technique leading to a reduction in the

temperature of coke removal would be beneficial for

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 4 6 9 - 0



48 I.V. Kozhevnikov et al. / Applied Catalysis A: General 214 (2001) 47–58

the regeneration of deactivated solid HPA catalysts.

Several known methods, such as solvent extraction,

including supercritical extraction with CO2 or SO2,

and oxidation with ozone [8,10], could be useful for

removing coke at lower temperatures. Modification

of solid acid catalysts by platinum group metals, e.g.

Pt or Pd, to enhance their regeneration is well known

[7,8]. This allows a significant reduction in the tem-

perature of coke gasification with oxygen. Only few

studies have dealt with the deactivation and regenera-

tion of solid HPA catalysts so far [11–16]. Recently,

we have communicated that the Pd doping can greatly

facilitate the regeneration of HPA catalysts by the

aerobic burning of coke [17].

This paper presents a detailed account of our studies

into the coking and regeneration of silica-supported

PW catalysts in the gas-phase conversion of hydro-

carbons. The oligomerisation of propene, previously

studied with HPA catalysts [11,12], was chosen as a

model reaction. The coked versus fresh catalysts werecharacterised by 31 P and 13C MAS NMR, XRD, XPS

and TGA/TPO. Various methods of coke removal such

as solvent extraction, ozone treatment and aerobic ox-

idation were explored. The effect of palladium doping

on the coke formation and burning was studied.

2. Experimental

2.1. Materials

Tungstophosphoric acid, H3PW12O40·nH2O, from

Aldrich, palladium acetate from Johnson Matthey andsilica Aerosil 300 from Degussa were used as pur-

chased. All solvents were analytical grade and distilled

before use.

2.2. Techniques

Magic-angle spinning (MAS) solid-state NMR

studies were carried out on a Bruker Avance DSX400

NMR spectrometer under ambient conditions. The31P NMR spectra were recorded at 161.99 MHz using

a 7 mm rotor probe with 85% phosphoric acid as an

external standard. The spinning rate was 4 kHz. The

1H–13C cross-polarisation MAS NMR spectra wererecorded at a frequency of 100.6 MHz. The peaks

were referenced to tetramethylsilane (TMS) as an

external standard. The spinning rate was 3–4kHz.

Catalyst samples after treatment were kept in a des-

iccator over P2O5 until the NMR measurements.

Thermogravimetric analysis (TGA) was performed

using a Perkin-Elmer TGA7 analyser. The carrier

gas was air and the samples were heated from 40

to 700◦C at a rate of 20◦Cmin−1. Performed in air,

TGA (TGA/TPO, temperature-programmed oxida-

tion), allows quantitative measurement of the amount

of deposited coke and the temperature of its aerobic

gasification [7,8,11,12]. The TGA/TPO analysis of

“soft” and “hard” coke was carried out as described

elsewhere [11,17], the cokes with the TGA/TPO

removal range of 170–370 and 370–570◦C referred

to as soft and hard coke, respectively.

XPS studies were carried out on an AMICUS XPS

spectrometer using Mg anode. The survey (wide) scans

were taken using a 1.0 eV step size and 272 ms dwell

time. The narrow scans were measured using a 0.1 eV

step size and a dwell time of 1293 ms.XRD studies were performed on a Phillips PW1390

diffractometer under ambient conditions. Untreated

catalyst samples were stored in a desiccator over

P2O5 prior to XRD measurements. The catalysts

treated under a particular atmosphere were measured

immediately after the treatment.

2.3. Catalyst preparation

PW/SiO2 catalysts containing 20 or 40 wt.% PW

were prepared by impregnating Aerosil 300 silica with

an excess of a methanolic solution of H3PW12O40 as

described elsewhere [18]. The catalysts were driedovernight at 120◦C and then powdered. The BET sur-

face areas were 285 and 140 m2 g−1 for 20 and 40%

PW/SiO2, respectively. Palladium-doped 20 wt.%

PW/SiO2 catalysts, containing 1.6–2.5 wt.% Pd, were

prepared by two techniques as described elsewhere

[17]: (1) by impregnating the PW/SiO2 catalyst with

a toluene solution of Pd(OAc)2 followed by evapora-

tion of toluene during which Pd(II) reduced to Pd(0)

and (2) by impregnating Pd/SiO2, prepared by loading

Aerosil 300 silica with Pd(OAc)2 followed by reduc-

tion of Pd(II) to Pd(0) with H2, with a methanolic

solution of H3PW12O40. The Pd-modified catalysts

were finally dried overnight at 120◦C. Both prepa-rations were found to yield similar catalysts with

respect to their coking and regeneration [17]. In this



I.V. Kozhevnikov et al. / Applied Catalysis A: General 214 (2001) 47–58 49

work, the Pd-modified catalysts prepared by the first

technique were mainly used.

2.4. Exposure of PW/SiO2 to solvent vapour

An amount of 40 wt.% PW/SiO2 was treated with

solvents at ambient temperature or at 150◦C. At am-

bient temperature, the catalyst (0.5 g) was placed in a

Pyrex boat and brought into contact with air saturated

with the vapour of a solvent in a closed glass vessel

for 18 h. The treatment at 150◦C was carried out in

a tubular furnace. The catalyst (0.5 g) placed in a

Pyrex boat was exposed to a flow of air (80 ml min−1)

containing 7 vol.% of solvent vapour for 4 h. A gas

bubbler placed in a water bath at a suitable temper-

ature was used for saturating air with solvents. After

solvent treatment, the catalyst was analysed by XRD.

2.5. Coking

Coking was performed in a fixed-bed flow reac-

tor under propene at atmospheric pressure. Prior to

coking, the catalyst (3.0 g) was pre-treated at 200◦C

under a 20 ml min−1 flow of dry nitrogen for a period

of 2 h. Then the sample was treated with propene at

20mlmin−1 and 200◦C for a certain period of time,

typically 1 h for soft coke and 2 h or more for hard

coke. (Hereafter “soft” and “hard” coke are referred

to those with the TGA removal range of 170–370

and 370–570◦C, respectively [7,11].) After that the

volatile hydrocarbons were removed from the cata-

lyst by purging with dry nitrogen at 20 ml min−1 and

200◦

C for 15 min.

2.6. Propene oligomerisation and catalyst

regeneration

The propene oligomerisation was carried out us-

ing a stainless steel tubular fixed-bed flow reactor

housed in a three zone SSL tubular furnace fitted with

Eurotherm temperature controllers, with on-line GC

analysis (a Varian 3800 Gas Chromatograph equipped

with TCD and FID detectors and a 30 m VH1 mega-

bore column). A gas mixture of propene and nitrogen

was passed through the catalyst bed using mass flow

controllers. A typical experiment was carried out asfollows. The catalyst (1 g) was activated by heating

under a nitrogen flow (30 ml min−1) for 2 h at 200◦C

in the fixed-bed reactor. Then a mixture of propene

and nitrogen, 1 and 49 mlmin−1, respectively, was

fed to the catalyst bed at 200◦C.

Over time, a strong deactivation of the catalyst by

coke deposition was observed. In the case of 2.5%

Pd-doped 20 wt.% PW/SiO2, the deactivated catalyst

was regenerated by aerobic burning of the coke as

follows. The reaction was continued for a period of

about 3 h then stopped, and the catalyst was cooled

down in a nitrogen flow (49 ml min−1). After that the

catalyst was regenerated at 350◦C in air (50 ml min−1)

for a period of 2 h. Better results were obtained when

the air treatment as above followed by the reduction

of the catalyst under a flow of 25% H2 in N2 at 225◦C

(50 mlmin−1) for 2 h.

2.7. Solvent extraction of coke

A coked catalyst was placed in a 50ml round-

bottomed flask fitted with a magnetic bar, and anorganic solvent (20 ml) was added. The mixture was

heated to reflux with stirring and held at reflux for 3 h.

Then the mixture was allowed to cool, and the solid

was isolated by filtration. Excess solvent was removed

from the solid at 150◦C under vacuum (10−2 mmHg)

over a period of 3 h.

2.8. Oxidation of coke with ozone

Coked catalysts (1.0 g) were treated with a gas flow

containing 6% ozone in oxygen in a glass tubular

fixed-bed flow reactor at 150◦C and a flow rate of 80

or 320 mlmin−

1. During the oxidation, decolourationof the catalysts occurred. The amount of coke was

measured by TGA.

3. Results and discussion

3.1. Coking PW/SiO2 catalysts

Passing a dry propene flow through the PW/SiO2

catalyst containing 20–40 wt.% PW at 150–200◦C

resulted in coke deposition on the catalyst surface,

the amount of coke and its nature dependent on the

temperature and time-on-stream, as demonstrated byTGA/TPO. Fig. 1 shows typical TGA/TPO data for

the 20 wt.% PW/SiO2 catalyst coked for 1 or 3 h at




Fig. 1. TGA/TPO for 20% PW/SiO2 coked with propene at 200◦C for (a) 1h and (b) 3h.

200◦C. As seen, the longer the time of coking, the

harder, i.e. more difficult to burn, the coke forms.

Sample (a) coked for 1 h shows a small peak at 230◦C

(0.9% weight loss), which can be attributed to low

molecular weight propene oligomers, referred to as

soft coke, and a large peak at 505◦C (4.0% weight

loss), representing higher aliphatic oligomers and pol-

yaromatics referred to as hard coke [11,17]. Sample

(b) coked for 3 h contains a harder coke, burning at a

higher temperature; it shows a major peak at 560◦C

(4.6% weight loss), apparently representing mainly

polyaromatics [17]. In this temperature range, the

fresh 20 wt.% PW/SiO2 catalyst shows only one peak at 450◦C (0.2% weight loss) due to the decomposi-

tion of HPA, releasing 1.5H2O per Keggin unit [17],

which is negligible compared to the weight loss for

the coked catalysts.

Fig. 2 shows the time course of coke formation on

20 wt.% PW/SiO2 at 200◦C. As seen, the coke builds

up quickly, reaching the amount of ca. 5 wt.% in about

1 h, followed by a slower deposition. This indicates

that the catalyst is rapidly deactivated by coking. It

is also seen that the nature of coke changes with the

time-on-stream: the amount of harder coke, with the

TGA/TPO removal range of 370–570◦C, increases at

the expense of softer coke with the TGA/TPO removalrange of 170–370◦C. The reduction in the fraction of

the softer coke with time suggests that the harder coke

is formed over time on the surface of the solid, prob-

ably from the rearrangement of the coke precursors

initially formed.

Addition of nucleophilic compounds such as water,

methanol, etc. to the propene flow caused the forma-

tion of oxygenated products at the expense of propene

oligomers, as expected. Quite unexpectedly, it was

found that the additives strongly inhibited coke for-

mation (Table 1). Thus, without additives, the 40 wt.%

PW/SiO2 catalyst made 3.6% coke in 3 h at 150◦C.

Fig. 2. Plot of the amount of coke (total, soft and hard) vs. time

for 20% PW/SiO2 coked with propene at 200◦C.




Table 1

Effect of additives (7 vol.%) to propene flow on coke formation

on 40 wt.% PW/SiO2 at 150◦C

Additive Time-on-stream (h) Amount of coke (%)

None 3.0 3.6

H2O 3.0 0.5

Methanol 3.0 1.7

Acetic acid 3.0 2.6

Addition of water to the propene flow greatly reduced

the amount of coke to 0.5%, isopropanol found to-

gether with propene oligomers among the products.

Addition of methanol brought the amount of coke

down to a half of that formed by pure propene, methyl

isopropyl ether being formed along with propene

oligomers. Acetic acid caused the least effect, although

isopropyl acetate was found among the products. The

additives are likely to change the catalyst activity as

well. At this stage, however, it is untimely to discuss

the effect of additives on coking versus catalyst activ-

ity as no activity measurement was done in this work.

To explain the above results we have studied the ef-

fect of these additives on the catalyst in the absence

of propene using XRD to monitor the state of HPA

in the catalyst. It was found that exposure of 40 wt.%

PW/SiO2 to an atmosphere of air saturated with wa-

ter, acetic acid, methanol or methyl acetate at ambient

temperature overnight completely destroyed the crys-

tallinity of HPA (Fig. 3). As all these solvents easily

dissolve PW, the loss of HPA crystallinity indicates

that after such treatment the HPA exists as a solution

intercalated in the pores of silica. In contrast, expo-sure of this catalyst at 150◦C to an air flow contain-

ing 7 vol.% of water, methanol or acetic acid for 4 h,

which is similar to the coking conditions, had practi-

cally no effect on the HPA, the Keggin structure and

the crystallinity remaining unchanged (Fig. 4). Appar-

ently at 150◦C, the amount of the solvents absorbed in

the catalyst is too small to affect the crystal structure

of HPA.

As the additives did not affect the structure of HPA

in the catalyst, their effect on the coke formation could

be explained as a result of their influence on (i) the

desorption of reaction products from the catalyst sur-

face, (ii) the acid strength of HPA proton sites or (iii)

the mechanism of propene conversion.

The co-feeding of water is commonly used to en-

hance the desorption of reaction products from the

catalyst. This often leads to an increase in reaction se-

lectivity, sometimes at the expense of activity. In our

case, the additives of water and other polar solvents

could facilitate the desorption of coke precursors from

the catalyst, decreasing the coke laydown.

On the other hand, the additives will affect the

acidity of HPA catalyst and, therefore, its activity.

This effect will depend on the basicity of the addi-

tives. The PW acid strength must be weaker in the

presence of water or methanol than in the presence of

acetic acid because the first two are much more ba-

sic. Hence, the catalytic activity of HPA towards thecoke formation, is expected to be lower with water or

methanol than with acetic acid, in agreement with the

experiment (Table 1). The same should apply to the

propene oligomerisation as well, for the catalyst ac-

tivity is likely to change parallel for oligomerisation

and coke formation.

The mechanism of acid-catalysed propene oligome-

risation can be adequately described as a carbenium-

ion one, including the formation of an isopropyl

carbocation type intermediate (probably as an ion

pair) by proton transfer from the catalyst to propene

[19,20]. Subsequent chain growth yields eventually a

range of propene oligomers together with coke as aby-product. In the presence of an acid catalyst, the

nucleophilic additives due to their high affinity to-

wards carbocations will interact with the isopropyl

carbocation to give oxygenated products at the ex-

pense of oligomers and coke. This is indeed the case,

water being the most active scavenger of the isopropyl

carbocation. The overall mechanism can be schema-

tically represented as follows:




Fig. 3. XRD patterns for (a) 40% PW/SiO2 as-made and after treatment at ambient temperature overnight with solvent vapour: (b) acetic

acid; (c) methyl acetate; (d) water; (e) methanol.

3.2. Characterisation of coked versus fresh catalysts

The 31P NMR spectra of the as-made PW/SiO2

samples showed a well-known single peak around

−15 ppm (Fig. 5a) which is associated with the

Keggin-type PW [18]. The Pd-doped catalysts exhib-

ited the same spectrum (Fig. 5b), indicating that the

doping does not affect the HPA structure. This is also

supported by XRD data: the XRD patterns were thesame for the undoped and 2.5% Pd-doped PW/SiO2

catalysts as well as for the bulk PW (Fig. 6). Note that

the crystallinity of supported HPA catalysts can vary

depending on the preparation conditions, especially

on drying (cf. Fig. 6b and c).

Coking of PW/SiO2 catalysts, both undoped and

Pd-doped, for 1 or 3 h did not change the 31P NMR

chemical shift, although some line broadening was

observed compared to the fresh catalysts (Fig. 5c and

d). The latter can be explained by the interaction of

HPA with coke. The significant line broadening of the31P NMR spectrum for PW supported on active carbon

has been reported [21]. These data indicate that the




Fig. 4. XRD patterns for 40% PW/SiO2

after treatment (150◦C, 4 h) with an air flow containing 7 vol.% of solvent vapour: (a) water; (b)

acetic acid; (c) methanol.

Keggin structure in the unmodified and Pd-modified

HPA catalysts is not destroyed by the formation of

coke.

The oxidation state of tungsten in the coked as

compared to fresh catalysts was examined by ex situ

XPS. The binding energies of W 4p7/2 and 4f 5/2 peaks

were found to be 37.9 ± 0.1 and 39.6 ± 0.1 eV, re-

spectively, typical of W(VI) [22] (Fig. 7). These were

practically the same for the fresh and coked undoped

and Pd-doped catalysts, indicating that the presence

of coke or Pd did not change the oxidation state of tungsten in the catalysts. It should be noted, however,

that, as the XPS measurements were performed ex situ,

re-oxidation of tungsten in the coked catalysts could

not be excluded.

The ex situ XPS of the 2.5% Pd-doped catalyst,

both fresh and coked, showed a doublet in the pal-

ladium region which can be assigned to Pd 3d5/2

and 3d3/2 [23], the binding energies being 338.0

and 343.2eV for the fresh catalyst and 337.8 and

342.9 eV for the coked, respectively, indicating the

similar state of palladium in both catalysts. Prelimi-

nary analysis of these spectra points to the presence

of both Pd(0) and Pd(II) in the catalysts, apparentlythe latter being formed by aerobic oxidation of Pd(0)

while exposed to air. The intensity of the peaks,




Fig. 5. 31P MAS NMR spectra for 20% PW/SiO2: (a) as-made; (b) 2.5% Pd-doped; (c) coked for 1 h at 200◦C; (d) coked for 3 h at 200◦C.

however, was too low for quantitative analysis. Simi-

lar observation of Pd(0) and Pd(II) by XPS in the par-tially reduced salt Pd3[PMo12O40]2 has been reported

[24].

Earlier we reported the 13C CP MAS NMR spec-

tra for coked PW/SiO2 catalysts [17]; these were

found different for the coked undoped and Pd-doped

PW/SiO2 catalysts. The undoped catalyst coked for 1

or 3 h showed a broad peak around 21 ppm referenced

to TMS which was attributed to aliphatic hydrocar-

bons. There was also another peak there, around

129 ppm, which was assigned to polyaromatic hydro-

carbons. The relative intensity of the aromatic peaks

was higher in the hard coked samples. The 2.5%

Pd-doped 20 wt.% PW/SiO2 catalysts coked for both1 and 3 h showed peaks which could only be assigned

to aliphatic hydrocarbons. Thus, both polyaromatic

and aliphatic coke form on the unmodified catalyst,

while Pd-doping inhibits the formation of polyaro-matics. The latter may be explained assuming that

palladium can promote hydrogen transfer between

coke precursors, thus facilitating the formation of the

aliphatic coke.

3.3. TGA/TPO measurements for Pd-doped catalysts

Our TGA/TPO data (Fig. 1) show that the aerobic

gasification of coke from the undoped PW/SiO2 cata-

lyst proceeds to completion in the temperature range

centred at 500–560◦C, i.e. well above the decompo-

sition temperature of HPA, which makes impossible

regeneration of the catalyst by this method.We have found that Pd doping allows significant

reduction in the temperature of coke burning [17].




Fig. 6. XRD patterns for (a) bulk PW, (b) 40% PW/SiO2 as-made and (c) 40% PW/SiO2 doped with 2.5% Pd.

Fig. 8 shows the TGA/TPO for the 1.6–2.5% Pd-dopedPW/SiO2 catalysts coked for 1 h at 200◦C. It can be

seen that the addition of Pd gradually decreases the

temperature of coke burning, down to 350◦C at 2.5%

Pd, that is ca. 100◦C below the temperature of HPA

decomposition. The effect of Pd doping appears to be

two-fold. On the one hand, the Pd can catalyse the

combustion of coke, on the other, it inhibits the for-

mation of hard polyaromatic coke (see above). Simi-

lar results were obtained for Pt/Al2O3 catalyst [7]. It

was assumed that either platinum catalyses the oxida-

tion of coke or coke deposited on the metal is different

from that on the alumina [7].

3.4. Catalyst regeneration by aerobic oxidation

Oligomerisation of propene was studied as a test

reaction for the deactivation/regeneration of the

Pd-modified catalysts. Product analysis using gas

chromatography showed the major products to be C12

to C18 oligomers. Similar results for the oligomeri-

sation of propene using HPAs have been reported

earlier [11].

The 2.5% Pd-doped 20% PW/SiO2 catalyst showed

a very high initial activity, followed by a rapid deacti-

vation (Fig. 9). The reaction was continued for a periodof about 3 h, and by that time the conversion dropped

to about 17%. Then the reaction was stopped, theFig. 7. XPS of the 20% PW/SiO2 catalyst: (a) fresh; (b) coked

(200◦C, 3 h).




Fig. 8. TGA/TPO for 20% PW/SiO2 coked with propene at 200◦C for 1 h: (a) undoped catalyst; (b) 1.6% Pd-doped; (c) 2.0% Pd-doped;

(d) 2.5% Pd-doped.

catalyst was regenerated at 350◦C in air for a period

of 2 h and rerun. In the second run the performance of

the catalyst was virtually the same as that in the first

run [17]. In contrast, the undoped PW/SiO2 catalyst

did not regain its activity after regeneration under the

above conditions. Even better results were obtained

Fig. 9. Catalyst performance of fresh and regenerated 2.5% Pd-doped 20% PW/SiO2 for propene oligomerisation.

when the Pd-doped catalyst was regenerated by air

treatment as above, followed by the reduction under a

flowof25%H2 in N2 at 225◦C for 2 h toconvertPd(II)

formed during the aerobic oxidation to Pd(0). Fig. 9

shows the performance of the fresh and regenerated

catalysts.




Table 2

Extraction of coke with refluxing DCM (3 h) from 20 wt.% PW/SiO2 coked with propene at 200◦C for 1, 2 or 3h

Time of coking (h) Amount of coke (%) Residual coke (%)

Hard cokea Soft cokeb Total Hard cokea Soft cokeb Total

1.0 3.9 1.0 4.9 2.6 0.2 2.8

2.0 4.3 0.9 5.2 3.4 0.5 3.9

3.0 4.6 0.7 5.3 3.7 0.6 4.3a TGA removal range: 170–370◦C.b TGA removal range: 370–570◦C.

3.5. Coke removal

Finally, we attempted the removal of coke from HPA

catalysts using solvent extraction and oxidation with

ozone. These methods have been described in the lit-

erature for the removal of carbonaceous deposits from

various catalysts [8].

3.5.1. Solvent extractionExtraction of coked HPA catalysts was attempted

with refluxing solvents at atmospheric pressure, the

residual coke determined by TGA. As solvents,

toluene, cyclohexane and dichloromethane (DCM)

were used, none of these dissolves PW. Of these sol-

vents, DCM showed better results. Table 2 presents

the data on DCM extraction of three samples of

20 wt.% PW/SiO2 coked for different period of time

(1, 2 or 3 h), with an increasing fraction of the hard

coke. For the most lightly coked catalyst (1 h), DCM

extraction removed 42% of the total coke content

(78% of the soft coke and 34% of the hard coke).

With the sample coked for 2 h, we observed removalof 25% of the total coke content (43% of the soft coke

and 22% of the hard coke). For the catalyst coked for

3 h, 19% of the total coke content was removed (14%

of the soft coke and 19% of the hard coke). Since the

HPA is not soluble in DCM, extraction of coke using

this solvent would have appeared to have some utility

for very lightly coked HPA catalysts.

3.5.2. Oxidation with ozone

Some success in the removal of coke from, e.g.

pentasil zeolite catalysts [25] and Pt-Re/Al2O3 [26]

at relatively low temperatures (T < 180◦C) has

been reported when ozone was used as the oxidant.Unlike oxygen, with ozone, the coke burning was

non-selective and there was no preferential burning

at the metal centres during coke removal [26]. Ozone

has been used to remove the organic surfactant at

250◦C in the synthesis of mesoporous MCM-41 type

zeolites [27].

To test the utility of ozone for the burning of coke

on the surface of silica-supported HPA catalysts, the

20 wt.% PW/SiO2 catalyst coked at 200◦C for 3h,

containing 5.3% of coke, was used. The method em-

ployed involved heating the coked catalyst under aflow of 6% ozone in oxygen (80ml min−1) at 150◦C

for 6 h. This resulted in a reduction of 40% of the total

coke content. As the coke was removed, the catalyst

was observed to become paler in colour. Some cata-

lyst particles however remained black. Increasing the

flow rate of O3 /O2 to 320ml min−1 improved the effi-

ciency of this process, with only a 5 h period necessary

to remove all coloration from the surface of the cat-

alyst, the residual amount of coke being <0.5%. No

breakdown of the HPA Keggin structure was observed

(31P NMR) after coke removal. Recoking of the re-

generated catalyst gave 5.0% coke deposition which

indicates nearly full recovery of catalyst activity.

4. Conclusions

The development of a technique leading to a

reduction in the temperature of coke removal is of

importance for regeneration of deactivated solid HPA

catalysts. The formation of coke during the oligomeri-

sation of propene, although rapidly deactivating the

catalyst, does not affect the Keggin structure of

silica-supported PW which justifies attempts to re-

generate such catalyst. Palladium doping of PW/SiO2

catalysts inhibits the formation of polyaromatic coke;only aliphatic coke, that appears easier to burn, is

detected. In contrast, the undoped catalysts form a




mixture of aliphatic and aromatic coke. Solvent ex-

traction of coke using DCM under reflux proved to be

relatively successful in removing both hard and soft

coke from very lightly coked HPA catalysts. Ozone

treatment can be used to clean up heavily coked HPA

catalysts at temperatures as low as 150◦C, completely

removing both hard and soft coke. This method,

whilst perhaps not of great practical interest, should

enable us to remove surface coke without destroying

the HPA, thus allowing to probe the acid sites after

catalyst regeneration. Most importantly, the aerobic

gasification of coke on Pd-modified PW/SiO2 occurs

at significantly lower temperatures than on the un-

doped PW/SiO2, which allows regeneration of the

catalyst without destroying the Keggin structure of

PW, hence without loss of its catalytic activity.

Acknowledgements

This work was supported by BP Amoco Chemicals

Ltd. We are indebted to Dr. H. He (Liverpool Univer-

sity) for measuring the NMR spectra and to Dr. A.

Roberts (Kratos Analytical) for measuring the XPS

spectra.

References

[1] I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171.

[2] Y. Izumi, K. Urabe, M. Onaka, Zeolite, Clay and Heteropoly

Acids in Organic Synthesis, Kodansha/VCH, Tokyo, 1992.

[3] I.V. Kozhevnikov, Catal. Rev. Sci. Eng. 37 (1995) 311.

[4] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996)

113.

[5] M. Misono, N. Nojiri, Appl. Catal. 64 (1990) 641.

[6] K. Sano, H. Uchida, S. Wakabayashi, Catal. Surv. Jpn. 3

(1999) 55.

[7] J. Barbier, Stud. Surf. Sci. Catal. 34 (1987) 1.

[8] E. Furimsky, F.E. Massoth, Catal. Today 17 (1993) 537.

[9] V.F. Chuvaev, K.I. Popov, V.I. Spitsyn, Dokl. Akad. Nauk.

SSSR 255 (1980) 892.

[10] M. Seapan, Z. Guohui, in: S.A. Bradley, M.J. Gattuso, R.J.

Bertolacini, Characterization and Catalyst Development, ACS

Symposium Series, Vol. 411, 1989 (Chapter 9).[11] J.S. Vaughan, C.T. O’Connor, J.C.Q. Fletcher, J. Catal. 147

(1994) 441.

[12] T. Hibi, K. Takahashi, T. Okuhara, M. Misono, Y. Yoneda,

Appl. Catal. 24 (1986) 69.

[13] A. Corma, A. Martinez, C. Martinez, J. Catal. 164 (1996)

422.

[14] T. Blasco, A. Corma, A. Martinez, P. Martinez-Escolano, J.

Catal. 177 (1998) 306.

[15] Y. Liu, K. Na, M. Misono, J. Mol. Catal. A 141 (1999) 145.

[16] B.B. Bardin, R.J. Davis, Appl. Catal. A 200 (2000) 219.

[17] M.R.H. Siddiqui, S. Holmes, H. He, W. Smith, E.N. Coker,

M.P. Atkins, I.V. Kozhevnikov, Catal. Lett. 66 (2000) 53.

[18] I.V. Kozhevnikov, K.R. Kloetstra, A. Sinnema, H.W.

Zandbergen, H. van Bekkum, J. Mol. Catal. A 114 (1996)

287.[19] J.M. Thomas, W.J. Thomas, Principles and Practice of

Heterogeneous Catalysts, VCH, Weinheim, 1997.

[20] A. Corma, Chem. Rev. 95 (1995) 559.

[21] I.V. Kozhevnikov, A. Sinnema, R.J.J. Jansen, H. van Bekkum,

Catal. Lett. 27 (1994) 187.

[22] B. Viswanathan, M.J. Omana, T.K. Varadarajan, Catal. Lett.

3 (1989) 217.

[23] P. Claus, H. Berndt, C. Mohr, J. Radnik, E.J. Shin, M.A.

Keane, J. Catal. 192 (2000) 88.

[24] M. Akimoto, K. Shima, H. Ikeda, E. Echigoya, J. Catal. 86

(1984) 173.

[25] R.G. Copperthwaite, G.J. Hutchings, P. Johston, S.W.

Orchard, J. Chem. Soc., Faraday Trans. 82 (1986) 1007.

[26] C.L. Pieck, E.L. Yablonski, J.M. Parera, Stud. Surf. Sci. Catal.

88 (1994) 289.

[27] M.T.J. Keene, R. Denoyel, P.L. Llewellyn, Chem. Commun.

(1998) 2203.

regenerar tungphosporic

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