regenerar tungphosporic
TRANSCRIPT
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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
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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
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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
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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.
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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:
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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
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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,
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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].
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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).
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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.
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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
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58 I.V. Kozhevnikov et al. / Applied Catalysis A: General 214 (2001) 47–58
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.
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