University of Groningen Oxidative dehydrogenation of ethylbenzene under industrially relevant conditions Zarubina, Valeriya IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2015 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Zarubina, V. (2015). Oxidative dehydrogenation of ethylbenzene under industrially relevant conditions: on the role of catalyst structure and texture on selectivity and stability. [S.l.]: [S.n.]. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 11-04-2020
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University of Groningen
Oxidative dehydrogenation of ethylbenzene under industrially relevant conditionsZarubina, Valeriya
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.
Document VersionPublisher's PDF, also known as Version of record
Publication date:2015
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):Zarubina, V. (2015). Oxidative dehydrogenation of ethylbenzene under industrially relevant conditions: onthe role of catalyst structure and texture on selectivity and stability. [S.l.]: [S.n.].
CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.
Oxidative dehydrogenation of ethylbenzene to styrene over MWCNT. Regeneration: myth or reality?
Selectivity and activity are two important aspects in the oxidative dehydrogenation (ODH) of
ethylbenzene (EB) to styrene. Besides those attributes, it is desirable that a catalyst is stable, or at
least a regenerable, that can be used for a number of runs. Inorganic-based catalysts are easier to
be regenerated by coking as the thermal stability is superior to carbon-based counterparts, though
changes in texture and active sites can occur. These aspects imply that the material will have a
limited life time, after that it must be recycled by a fresh batch. Despite the practical importance of
stability, it is hardly found any study on regeneration of EB ODH-based catalysts.
In this work, two representative ODH-based catalysts (-Al2O3 and MWCNTs) have been studied
regarding their regeneration towards coke removal by calcination. Before regeneration, the
catalysts were exposed to EB ODH reaction conditions. Various regeneration strategies were
investigated and the derived materials’ properties were characterized by physico-chemical
methods, and those properties are compared to the fresh counterparts.
Chapter 5
110
5.1 Introduction
Direct dehydrogenation of ethylbenzene (EB) in the presence of steam is the main way to produce styrene in industry. About 85% of styrene (ST) is produced by this way over a K-promoted Fe2O3 catalyst which is highly selective to styrene and stable
[1,2]. The process takes place at temperatures as high as 580-630 °C [1], due to the endothermic thermodynamics and therefore suffers from high energy consumption. The conversion per pass is low due to the equilibrium limitations,
involving a large recycle, and this has been the major driver to develop alternative ST production routes.
One of the aspects which can be improved is reducing the large amount of
steam in the direct dehydrogenation reaction. Many efforts have been put forward to improve the process by making it steam-free and applying lower temperatures. The use of oxidants such as O2 [3,4] is very promising as it helps to shift the reaction by
H2O formation. Therefore the oxidative dehydrogenations (ODH) have been thoroughly investigated, but it is not commercialized yet due to the limited catalyst stability and low selectivity to styrene.
The O2-based oxidative pathway was originally studied on oxide-based carriers such as alumina [5]. It was shown that the coke deposited on the alumina’s surface under the oxidative conditions promotes the activity and selectivity [6-9] due to
presence of Lewis acid sites. It was further proposed [10-13], and recently supported [14-16], that the formed coke acts as a selective and active site. Such type of coke has relatively high oxygen content (O:C=0.10-0.15 at.) and is considered to be
polyaromatic. Another family of catalysts that drew significant attention for the O2-based EB
oxidative dehydrogenation to styrene are carbon-based materials. Many types of
carbon-based structures have been claimed to be active and selective, including activated carbons [17-30], onion-like carbons [31-33], carbon nanofibers [34-41], carbon nanofilaments [34], diamonds [28,30,33], graphite’s [19,34,38,42], MWCNT
[28,30,40,32,33,42-49] , and other types of carbons [50-52]. It is known that some of those materials, e.g. activated carbons, gasify/decompose under the reaction conditions [22,23,28,45]. The most stable ones are MWCNT. In a previous study, it
was shown that it has a slow catalyst deactivation and good thermal stability under harsh conditions with the deposition of reaction-based coke [53]. Thus, MWCNT is a promising catalyst for the alternative ODH process.
Besides of activity and selectivity, it is desirable that a catalyst is stable, or at least a regenerable, that can be used for a number of runs. Inorganic-based
catalysts are easier to be regenerated by coking as the thermal stability is high, though changes in texture and active sites can occur. These aspects imply that the material will have a life time, after that it must be recycled by a fresh batch. Despite
the practical importance of stability, it is hardly found any study on regeneration of EB ODH-based catalysts.
In this work, two representative spent ODH-based catalysts (-Al2O3 and
MWCNTs) have been studied regarding their regeneration towards coke removal by
calcination. Before regeneration, the catalysts were tested under EB ODH conditions. Various regeneration strategies were investigated and the derived materials were characterized by physico-chemical methods and the properties are compared to the
fresh counterparts.
Oxidative dehydrogenation of ethylbenzene to styrene over MWCNT. Regeneration: myth or reality?
111
5.2 Experimental methods
5.2.1 Materials
In this study a low SiO2-stabilized -Al2O3 extrudates (Albemarle Catalysts BV,
denoted as GA-F) and purified MWCNT (Hyperion CS-02C-063-XD, denoted as MW-F) were employed. The extrudates were crushed and sieved into a 212-425 μm fraction
used for the catalytic tests and characterization
5.2.2 Regeneration
Two regeneration protocols were carried out on the spent materials, mild and conventional. The mild regeneration (denoted as MR) was carried out in a quartz-
tube housed tubular oven (Nabertherm RT 50/250-11). The sample was loaded in a flat quartz-based crucible that was placed horizontally in the centre of the furnace’s isothermal heating zone. The spent MWCNT (denoted as MW-S) and spent -alumina
(GA-S) were regenerated at temperature at 450 °C in 1% vol. O2/Ar at a heating rate of 3 °C.min-1 and held for 2, 5, 8, and 24 h for MWCNT. In the case of -alumina
5 and 24 h were applied. The conventional regeneration (denoted as R) was applied
on the spent -alumina; this was done in the in LT9/11 Nabertherm box furnace at
450 °C in air at a heating rate of 3 °C.min-1 and held for 5 h. Fresh, spent and regenerated materials with their codes and treatments are given in Table 1.
Table 1. Materials and their treatments
Sample code a Material Treatment
GA-F -Al2O3 as-received
GA-S -Al2O3 spent after reaction test, 60 h TOS
GA-R5 -Al2O3 spent regeneration in air, 5 h
GA-MR5 -Al2O3 spent mild regeneration in 1 %O2/Ar, 5 h
GA-MR24 -Al2O3 spent mild regeneration in 1 %O2/Ar, 24 h
MW-F MWCNT as-received
MW-S MWCNT spent after reaction test, 60h TOS MW-MR2 MWCNT spent mild regeneration in 1%O2/Ar, 2 h
MW-MR5 MWCNT spent mild regeneration in 1%O2/Ar, 5 h MW-MR8 MWCNT spent mild regeneration in 1%O2/Ar, 8 h MW-MR24 MWCNT spent mild regeneration in 1%O2/Ar, 24 h
a. The suffix is related to the treatment: F (fresh), S (spent after the reaction cycle); R means conventional regeneration and MR mild regeneration; the number indicates the number of hours after reaching 450 °C.
5.2.3 Catalysts characterization
The organic content of the fresh and spent catalysts was quantified by thermogravimetric analysis (TGA) on a Mettler-Toledo analyzer (TGA/SDTA851e).
The weight loss was monitored for a temperature program from 30 to 1000 °C at a heating rate of 10 °C/min using a flow of synthetic air of 100 mL.min-1 NTP. Blank curve subtraction using an empty crucible was taken into account. The oxidation rate
patterns (TPO) were obtained in the same instrument making use of the TGA derivative patterns.
Chapter 5
112
The textural properties of the catalysts and the bare supports were analyzed
by N2-physisorption at -196°C using a Micromeritics ASAP 2420. Fresh and regenerated -alumina samples were degassed at 300 oC for 10 h under vacuum.
Spent samples were degassed at 200 °C for 10 h to ensure that the coke deposited
on -alumina and carbon of MWCNT are not altered during the degassing. The
surface area (SBET) was calculated with the standard BET method [54] in the relative pressure range 0-0.25. The pore volume (VT) was calculated using the single point
total desorption pore volume at the relative pressure 0.98. Pore size distributions were calculated using the BJH-model [55].
Raman spectra were obtained with 785 nm excitation line, 30 mW on the
Perkin Elmer Ramanstation 400 spectrometer.
5.2.4 Catalytic tests
The catalytic tests were performed in a setup with six parallel quartz fixed bed reactors (inner diameter 4 mm) in down-flow operation. The reactors were loaded from top to bottom with a quartz wool plug, 10 cm glass beads (0.5 mm diameter),
and 65 mm catalyst bed (0.80 ml) to ensure that the catalyst bed is located in the isothermal zone of the furnace. The glass beads have limited conversion which is less than 3% EB conversion under all applied conditions.
Each reactor gas feed has a flow of 36 ml/min (NTP) and consist of a mixture of nitrogen, oxygen, and ethylbenzene. A liquid ethylbenzene flow of 1 g.h-1 evaporates (3.6 ml.min-1 vapour at NTP) resulting in the 1:10 volume ratio of
ethylbenzene and gas (10 vol.% EB) with a GHSV of 3000 l/l/h. The EB liquid evaporates in a -Al2O3 filled tube in a synchronized flow with the gas feed. Pressure
in the reactor system was 1.2-1.3 bars and an atmospheric outlet pressure drop was
typically 0.2-0.3 bars. The reactor outlet flows were analyzed using an online two channel gas
chromatograph with a TCD (columns: 0.3m Hayesep Q 80-100 mesh with back-flush,
25m×0.53mm Porabond Q, 15 m × 0.53mm molsieve 5Å) for permanent gasses analysis (CO2, H2, N2, O2, CO) and a FID column (30 m × 0.53 mm, Df = 3 mm, RTX-1) for hydrocarbon analysis (methane, ethane, ethene, benzene, toluene,
ethylbenzene, styrene, and heavy aromatics). The catalytic test was carried out at various temperatures (475, 450, 425, and 450 °C) and O2/EB = 0.6 and 0.2 (vol.). For all EB conversion data the oxygen conversion is 100%, unless otherwise is
stated. All characterizations for the spent catalysts were done after the complete testing cycle of 70 h.
5.2.5 Regeneration efficiency of the carbon-based materials
The regeneration efficiency of the carbon-based materials was defined via the
textural parameters (
, eq. 1) as the ratio between the final surface area,
corrected by the weight loss during regeneration, to the initial surface area, which
means the fraction of surface area that is recovered after regeneration or fraction that is lost, in case that this concept is applied to the spent catalyst:
[
]
(1)
where
is the BET surface area of the regenerated material,
is the
corresponding BET of the fresh sample and is the weight loss during
regeneration.
Oxidative dehydrogenation of ethylbenzene to styrene over MWCNT. Regeneration: myth or reality?
113
The total textural efficiency ( ) contains the weight loss during the
reaction:
[
] [
]
(2)
is the weight loss or gain during the ODH reaction. For MWCNT in the applied
catalyst run .
5.2.6 Regeneration efficiency of the alumina-based materials
This is defined as the corrected weight loss, relative to the bare alumina, from the TGA patterns as:
(
) (3)
where superscript ‘x’ refers to any material (i.e. spent, regenerated, fresh). The
recovery of the texture can be defined as:
(4)
5.3 Results
5.3.1 Catalytic tests
Figure 1 shows the catalyst performance of MWCNT (MW-F) and -alumina (GA-F)
catalysts under relevant conditions, meaning a high EB concentration and limited O2
with 20% excess compared to the stoichiometric amount. EB conversion, ST, COx selectivity, and ST yield are plotted as a function of the time on stream (TOS). MWCNT is a readily active and selective catalyst at 475 oC, but -alumina requires
approximately five hours to be active. However the conversion and selectivity of -
alumina are higher. At the second temperature range at 450 °C during 20-40 h TOS, both catalysts have identical performance. The lowest temperature of the cycle (425 °C) shows higher activity and selectivity of MWCNT in comparison with -alumina,
which makes it attractive for low-temperature ODH of EB to styrene in comparison with -alumina. At the end of the catalytic cycle, at 450 °C, conversion and
selectivity are identical, and in agreement with step 2, though the conversion and selectivity values are lower due to deactivation. So the comparison of two steps at
450 °C shows that both catalysts deactivate in 2% for MWCNT and 5% for -alumina.
Both catalysts demonstrate moderate stability, and MWCNT still have some advantages at low reaction temperature; higher activity and selectivity than alumina-
based catalyst. The selectivity to COx is inversely coupled to ST (Fig. 1-c). This is because the selectivity to benzene/toluene and heavy condensates are much lower than COx/ST and independent of the applied reaction conditions.
Chapter 5
114
Figure 1. Time on stream EB conversion (a), selectivity to ST (b), selectivity to COx (c), and ST yield (d) at various temperatures (475, 450, 425, and 450 oC) and O2/EB= 0.6 and 0.2
(vol.); GHSV of 3000 l/l/h; 10 vol. % EB.
5.3.2 Spent catalysts
Temperature programmed oxidation was performed on the fresh and spent -alumina
and MWCNT materials for the evaluation of the coke burning profiles (Fig. 2). The
TPO profile of fresh MWCNT shows the presence of two types of coke which start to decompose at 400 °C and ends at 635 °C, with a maximum at 560 °C. It indicates a high thermal stability of the fresh MWCNT (Figure 2-b). TPO for the spent -alumina
starts to decompose earlier at 400 °C, and it ends at 520 °C with a maximum at 460
°C (Figure 2-a) in a single step. Spent MWCNT has two contributions (I and II), which includes the multi-walled backbone (peak II), while the low-temperature peak
(I) comes from a new type of carbon-based material that is deposited during the reaction (Figure 2-b), and it is ascribed to ODH-based polyaromatic coke.
425 450450
0.6 0.2 0.6 0.60.20.6 0.2
475
0 10 20 30 40 50 60 700
10
20
30
40
50
60
GA-F
MW-F
EB
Co
nve
rsio
n /
%
Time / h
0 10 20 30 40 50 60 7010
20
30
40
50
60
70
80
90
100
GA-F
MW-F
Se
lectivity t
o S
T /
%
Time / h
425 450450
0.6 0.2 0.6 0.60.20.6 0.2
475
425 450450
0.6 0.2 0.6 0.60.20.6 0.2
475
0 10 20 30 40 50 60 700
10
20
30
40
50
60
70
80
90
100
GA-F
MW-F
Se
lectivity t
o C
Ox / %
Time / h
425 450450
0.6 0.2 0.6 0.60.20.6 0.2
475
0 10 20 30 40 50 60 700
10
20
30
40
50
60
GA-F
MW-FS
T Y
ield
/ %
Time / h
a b
c d
Oxidative dehydrogenation of ethylbenzene to styrene over MWCNT. Regeneration: myth or reality?
115
The coke nature was investigated by Raman spectroscopy. The comparison of Raman spectra between fresh and spent MWCNT are shown in the Figure 3. The spectrum of the raw MWCNT material reveals two broad absorptions centered at
1600 and 1310 cm-1, which are indicative of amorphous carbon (G and D peaks). Raman spectrum of the spent catalyst indicates the presence of a more ordered structure that has been formed during the reaction. The D/G peak intensity becomes
smaller and this can be explained by two possible effects: a) removal of the more amorphous carbon domains by gasification/burning and b) deposition of sp2-rich carbon materials, or both effects; the latter is due to the buildup of new polyaromatic
ODH coke.
Figure 2. Oxidation rate patterns (TPO) for the fresh and spent: a) -alumina and b)
MWCNTs. = (Wo-W)/Wo; where Wo is the initial weight. Conditions: synthetic air, 100
ml.min-1, heating rate of 3 °C.min-1.
300 400 500 600 700 8000,0
0,2
0,4
0,6
a GA-F
GA-S1
Temperature / oC
- d
/dT
/ (
%.
oC
-1)
300 400 500 600 700 8000,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
b MW-F
MW-S1
Temperature / oC
- d
/dT
/
(%
. oC
-1)
a
b
I
II
Chapter 5
116
Figure 3. Raman spectra for the fresh (bottom) and spent (top) MWCNTs.
The textural properties of the fresh and spent -alumina (Fig. 4) and MWCNT
(Fig. 5) samples were investigated by N2-physisorption analysis. The isotherm of fresh -alumina has a type IV with hysteresis H1 [56]. This type represents
cylindrical pore geometry of solid particles with pore size equability and superficial
pore connectivity. The hysteresis changes to H2 type for the spent -alumina (Fig. 4)
with a closure point at 0.45 relative pressure. Hysteresis H2 [56] occurs in solids, where the pores have narrow necks and wide bodies or when the porous material has interconnected pores. The original material has no interconnectivity at 0.45
relative pressure, thus the pore neck restrictions of the spent sample is due to the excessive coke build-up. Pore size distribution curves of spent samples shift slightly
towards low pore sizes that also indicates pore blockage. The specific surface area is depleted from 272 (GA-F) to 152-154 m2.g-1 (spent, Table 2) which is ~44% reduction. The spent sample was measured twice to ensure that sample is
homogeneous; this was in agreement with TGA in terms of identical DTGA peak positions (Fig. 6-a, b) and residual weight losses (Table 2).
2000 1800 1600 1400 1200 1000 800
x5
D MWCNT fresh
MWCNT spent
Inte
nsity / a
.u.
Wavenumber / cm-1
G
Oxidative dehydrogenation of ethylbenzene to styrene over MWCNT. Regeneration: myth or reality?
117
Figure 4. Nitrogen sorption isotherms at -196 oC for the fresh and spent Alumina samples. Inset: BJH pore size distribution.
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,00
50
100
150
200
250
300
350
400
450
10 20 30 400,0
0,5
1,0
1,5
2,0
Pore width / nm
GA-F
GA-S1
GA-S2
Qua
ntity
Adsorb
ed (
cm
³/g
ST
P)
Relative Pressure (p/p°)
Figure 5. Nitrogen sorption isotherms at -196 oC for the fresh and spent MWCNT materials.
Inset: BJH pore size distribution.
0,5 0,6 0,7 0,8 0,9 1,00
200
400
600
800
0 20 40 60 80 100 120 140 1600,0
0,5
1,0
1,5
Pore width / nm
MW-F
MW-S1
MW-S2
Qua
ntity
Adsorb
ed (
cm
³/g
ST
P)
Relative Pressure (p/p°)
Chapter 5
118
Figure 6. TGA curves of two spent -alumina (top) and two spent MWCNT (bottom). Conditions:
synthetic air, 100 ml.min-1, heating rate of 3 °C.min-1. A) GA-S1; b) GA-S2; c) MW-S1; d) MW-S2. Peak I: ODH-based coke; peak II: MWCNT backbone.
Table 2. Texture properties of fresh, spent, and regenerated -alumina samples.
GA-MR5 5.8 0.93 b 160 (59) 0.341 (53) 0.59 GA-MR24 3.3 1.02 b 256 (94) 0.650 (102) 0.94
a. Determined by TGA weight loss between 200-800 oC; b. Spent GA-S1 was employed for the
regeneration study; c. between parentheses the percentage of recovery is given, relative to the fresh counterpart.
Figure 5 shows the isotherms and pore size distribution of fresh and spent MWCNT. Fresh MWCNT has an isotherm type IV with hysteresis H1, which characterizes cylindrical pore geometry and relatively high pore size uniformity. Pore
size distribution of the fresh MWCNT shows the presence of two peaks (Fig. 5, inset). The broad peak with a maximum of 30 nm is related to inter-rod mesopores and the small peak at 5 nm that can be related to smaller mesopores, likely from internal
open tubes. Spent samples show much less adsorption capacity, due to the excessive coke build-up. In contrast to -alumina, the shape of the isotherm and hysteresis
remain identical, of type IV H1, while the small peak at 5 nm pore size disappears. It
200 400 600 8000
20
40
60
80
100
-0,4
-0,2
0,0
0,2
0,4
re
lative w
eig
ht / %
Temperature / oC
derivative
/ %
(oC
)-1
derivative
200 400 600 8000
20
40
60
80
100
-0,4
-0,2
0,0
0,2
0,4
rela
tive w
eig
ht / %
Temperature / oC
derivative
/ %
(oC
)-1
derivative
200 400 600 8000
20
40
60
80
100
-1,0
-0,5
0,0
0,5
rela
tive
we
igh
t
/
%
Temperature / oC
de
riva
tive
/
%(o
C)-1
derivative
200 400 600 8000
20
40
60
80
100
-1,0
-0,5
0,0
0,5
rela
tive w
eig
ht / %
Temperature / oC
derivative
/ %
(oC
)-1derivative
a b
dc
I II I II
Oxidative dehydrogenation of ethylbenzene to styrene over MWCNT. Regeneration: myth or reality?
119
all indicates that the mesopores are blocked by the formed ODH coke during the reaction; however there is no indication of pore restrictions, due to the absence of low closure point. A duplo of the spent MWCNT was measured to account for sample
homogeneity. The homogeneity of the spent MWCNT is lower than for -alumina; the
BET varied in 7% while the pore volume in 16% (Table 3). This can be related to differences in ODH coke deposition along the reactor. Sample MW-S1 was employed
for the regeneration studies. TGA patterns (Fig. 6) show two combustion steps; the low-temperature process (type I) is due to the ODH-coke, while the higher temperature step originates from the burning of the MWCNT backbone, as discussed
in Fig. 2-b. The comparison of the DTGA curves indicates that MW-S2 has a slightly higher fraction of the ODH coke than MW-S1, which is consistent with the lower BET and pore volume (Table 3).
Table 3. Texture properties of fresh, spent, and regenerated MWCNT samples.
Material SBET (m2.g-1)a VT (cm3.g-1)a Wloss (%) (-)b
MW-MR24 502 (124) 1.239 (112) 58 0.52 1.24 a. Between parentheses the percentage of recovery is given, relative to the fresh counterpart; b. In parenthesis, the regeneration efficiency of the pore volume is given.
5.3.3 Regeneration studies
For the regeneration of MWCNT, different regeneration times under mild conditions
were applied to evaluate the recovery of the textural properties similar to the original material. Figure 7 shows a comparison between various regenerated MWCNT together with the fresh and spent materials. It is visible from the graph that MW-
MR2 sample has higher adsorption capacity, a trend that continues with MR5 and MR8. Thus, increasing the regeneration time at the applied oxygen partial pressure, gives possibilities to reach the textural properties of the original material.
Remarkably, 24 h of regeneration (MW-MR24) produces a material with better textural properties than the fresh MWCNT. This is also seen with a more intense PSD
than fresh MWCNT, while the PSD maximum shifts towards higher sizes. In general, the pore size distribution after regeneration increases with increasing the regeneration time and the small peak at 5 nm appears back for the regenerated
samples, except of MW-MR2. So, it demonstrates that 2 h is not enough to perform a good regeneration of the MWCNT. The derived textural properties are presented in the Table 3. It can be seen from the table that the spent MWCNT samples have 0.3
times its original surface area and ca. 0.4 times of the original pore volume. The BET of the regenerated increases notably and at 5 h (MR5) the BET surface area surpasses that for the raw material up to a factor 1.24 for the 24 h-regenerated
sample. The pore volume is also recovered, but it does not arrive to the original MWCNT’s quantity, with the exception of the 24 h that is a factor 1.12. The situation with the MR24 can be attributed to the fact that the regeneration can also clean the
MWCNT backbone by removing amorphous material; therefore the final material is more porous. Thus, from this study, it is concluded that both, BET and pore volume can be recovered after a mild regeneration. To make the comparison complete, the
weight loss due to the regeneration itself must be considered. This is considered in
Chapter 5
120
equation 1, where the regeneration efficiency based on the BET surface area and
weight loss is defined. When using this parameter, an optimal value is found. The regeneration efficiency increases from 0.77 (MR2) with a maximum of 0.88 at 5 h regeneration, and then decreases for 8 and 24 h. This information indicates that
because of the weight loss, the surface area cannot come back to the original value due to the weight loss; it also concludes that the optimal conditions in terms of BET surface area are 5 h. A second factor that affects the overall regeneration efficiency
is the weight loss associated to the ODH reaction itself; the total regeneration efficiency is defined in eq. 2 that accounts for the reaction weight loss. The consequence of this is the lower regeneration efficiency with a maximum of 0.70 for
MR5. This means that under the optimal conditions of regeneration, 30% of the total surface area is lost due to the reaction plus regeneration.
Figure 8 shows the oxidation rate patterns for the fresh, spent, and mild-regenerated samples for the MWCNT. It is clear from the figure that the peak of the spent catalyst at ca. 520 oC disappears with the increasing regeneration time. The
first peak of the spent catalyst disappears completely after regeneration for 24 h. The maximum of the regenerated samples’ peak shifts to 580 oC. It demonstrates that the coke which was formed on the MWCNT is removed upon regeneration, while
the MWCNT structure remains unmodified with improved thermal stability. The regeneration was also studied regarding the -alumina after the ODH
reaction under identical conditions. It is expected that -alumina can be easily
regenerated. The isotherms and pore size distribution curves of GA-R5 and GA-MR24
are nearly identical to the fresh -alumina (Fig. 9), meanwhile the isotherm and PSD
for GA-MR5 show less adsorption due to the incomplete regeneration. The TGA-based regeneration efficiency for the MR5 was about 93% but this 7% left residue
impacted on the BET with 41% lower than the fresh counterpart (Table 3). However, the regeneration in air for 5 h or 24 h in air gives rise to 95 and 94% recovery of BET surface area and approximately 100% pore volume’s recovery, with no residual
coke (Table 3).
Figure 7. Nitrogen sorption isotherms at -196 oC for the fresh, spent, and regenerated MWCNT at different conditions. See Table 1 for description of the treatment conditions. Inset: BJH pore size distribution.
0,5 0,6 0,7 0,8 0,9 1,00
200
400
600
800
1000
1200
0 20 40 60 80 100 120 140 1600,0
0,5
1,0
1,5
2,0
Pore width / nm
MW-F
MW-S1
MW-MR2
MW-MR5
MW-MR8
MW-MR24
Qua
ntity
Adsorb
ed (
cm
³/g
ST
P)
Relative Pressure (p/p°)
Oxidative dehydrogenation of ethylbenzene to styrene over MWCNT. Regeneration: myth or reality?
121
Figure 9. Nitrogen sorption isotherms at -196 oC for the fresh, spent, and mild regenerated -alumina. Inset: BJH pore size distribution.
5.4 Conclusions
MWCNT and -alumina show similar performance in the oxidative dehydrogenation of
ethylbenzene to styrene and demonstrate good stability under the applied
conditions; the selectivity is however relatively lower compared to the conventional process. Both types of materials suffer from coke deposition, under the reaction conditions, that depletes the textural features notably. -alumina can be fully
regenerated by calcination in air, or longer times under milder conditions, with the
0,5 0,6 0,7 0,8 0,9 1,00
50
100
150
200
250
300
350
400
450
500
0 10 20 300,0
0,5
1,0
1,5
2,0
Pore width / nm
GA-F
GA-S1
GA-R5
GA-MR5
GA-MR24
Qua
ntity
Adsorb
ed (
cm
³/g
ST
P)
Relative Pressure (p/p°)
Figure 8. Oxidation rate patterns (TPO) for the fresh, spent, and mild-regenerated samples for the MWCNT. = (Wo-W)/Wo; where Wo is the initial weight. Conditions: synthetic air, 100
ml.min-1, heating rate of 3 °C.min-1.
300 400 500 600 700 8000,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
MW-F
MW-S1
MW-MR2
MW-MR5
MW-MR24
Temperature / oC
- d
/dT
/ (
%.
oC
-1)
Chapter 5
122
complete recovery of its textural features and no residual coke. MWCNT can also be
regenerated under mild conditions. This was possible because the deposited ODH coke has a lower reactivity, i.e. appears in the TPO profiles at lower temperatures, than the MWCNT backbone. In order to quantify the regeneration efficiency, the
weight loss during the regeneration and reaction conditions have been taken into account. At the optimal conditions, 70% of its initial surface area can be recovered. It was also found that the specific surface area, for prolonged regeneration times,
can be even higher than the starting material. This was explained by the cleaning effect of the MWCNT backbone from amorphous domains, at the expense of a substantial weight loss. Therefore, MWCNT can not be considered a single-use
material, but can be partially recycled.
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