Synthesis of nano/micro scale ZSM-5 from kaolin and its catalytic performance

Article abstract of DOI:10.1134/S0023158417050184

Nano/micro scale ZSM-5 zeolites were synthesized by using natural kaolin as raw material. The effect of particle size on the catalytic performance of ZSM-5 zeolite for the methanol to olefins conversion was evaluated in a fixed-bed reactor. The results indicated the crystal size had a significant effect on the catalytic stability and the products distribution. ZSM-5 with nanosize showed better tolerance to coke formation, longer catalytic lifetime, and higher selectivity to propylene. The selectivity to propylene on nanosized ZSM-5 was on average 4.5% higher than on the submicron sample and 10% higher than on microsized ZSM-5. After the reaction was conducted for 20 h the ZSM-5 catalyst synthesized from kaolin showed longer lifetime and higher propylene selectivity than the sample synthesized with chemical materials The reason can be explained by the occurence of such elements as Fe, P, and especially Ti.

Full text of DOI:10.1134/S0023158417050184

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 5, pp. 541–548. © Pleiades Publishing, Ltd., 2017.
Synthesis of Nano/Micro Scale ZSM-5 from Kaolin
and Its Catalytic Performance1
Feng Pana, Xuchen Lua, b, *, Yan Yana, and Tizhuang Wanga
aState Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences,
Beijing, 100190 P.R. China
bUnited Research Center for Resource and Materials, Wuhai, 016000 P.R. China
*e-mail: xclu@home.ipe.ac.cn
Received July 13, 2016
Abstracts—Nano/micro scale ZSM-5 zeolites were synthesized by using natural kaolin as raw material. The
effect of particle size on the catalytic performance of ZSM-5 zeolite for the methanol to olefins conversion
was evaluated in a fixed-bed reactor. The results indicated the crystal size had a significant effect on the cat-
alytic stability and the products distribution. ZSM-5 with nanosize showed better tolerance to coke forma-
tion, longer catalytic lifetime, and higher selectivity to propylene. The selectivity to propylene on nanosized
ZSM-5 was on average 4.5% higher than on the submicron sample and 10% higher than on microsized
ZSM-5. After the reaction was conducted for 20 h the ZSM-5 catalyst synthesized from kaolin showed longer
lifetime and higher propylene selectivity than the sample synthesized with chemical materials The reason can
be explained by the occurence of such elements as Fe, P, and especially Ti.
Keywords: ZSM-5 zeolites, crystal size, methanol to propylene, kaolin clay
DOI: 10.1134/S0023158417050184
1. INTRODUCTION
showed higher activity, greater stability, and higher
selectivity to propylene compared with the microsize
sample. Moreover, the introduction of some irons or
oxides also was extensively investigated. Zhao [10]
loaded phosphorus (P) on HZSM-5 to eliminate part
of the strong acid sites and the resulted P-modified
HZSM-5 showed excellent anti-coking ability and
propylene selectivity. Zhang [11] introduced many
kinds of ions such as Na, Li, Mn, and Fe by impregna-
tion method, the obtained catalysts showed high activ-
ity, improved selectivity in the MTO reaction and the
excellent regeneration performance. Rostamizadeh
[12] synthesized HZSM-5 catalyst and impregnated it
with different irons (P, Mn, Mg and Cs), the results
indicated that in the presence of Mn-ZSM-5 the high-
As one of the most important intermediates in pet-
rochemical industry, propylene could be produced by
the petroleum cracking process and the methanol
dehydration [1, 2]. Since the reserves of coal and nat-
ural gas are much more abundant than supply of oil,
and the methanol can be industrially produced from
natural gas and coal, the methanol-to-propylene
(MTP) process is a promising alternative to the oil
processing. Therefore, many researchers have focused
their attention on exploring the reaction mechanism
and optimizing the reaction conditions on ZSM-5 and
SAPO zeolites [3, 4].
Obviously, the performance of the catalysts and its
synthesis cost are the key to the MTO process. It is est methanol conversion (100%), long term lifetime
well known that coke formation in/on the acid sites (120 h) and the highest propylene selectivity (58.4%)
and channels causes the deactivation of ZSM-5 during
the MTP process. To prevent the formation of coke,
shortening the residence time by reducing the crystal
size and adjusting the acid strength with metal/non-
metal irons were usually adopted [1, 5, 6]. For
instance, Firoozi [5], Herrmann [7], Prinz and
Riekert [8], Bleken [9] et al. investigated the influence
of crystal size of the ZSM-5 zeolite on conversion of
methanol to olefins/propylene (MTO/MTP). They
made the conclusion that the nanosized HZSM-5
were achieved. Attempts were described to introduce
other elements such as B [13], Me (Me = Gr, Fe, Ni,
Ga, Ce, V, Mo and W) [14, 15].
Generally, ZSM-5 can be synthesized from natu-
rally occuring deposits (such as kaolin clay) as raw
materials. Some metallic elements involved in clay
minerals (such as Fe, Ti, etc.) can be incorporated into
the framework of zeolites during the synthesis process
[16]. Therefore, the physical and chemical properties
of zeolites obtained from natural clay are significantly
different from those obtained from chemical materi-
1
The article is published in the original.
541

542
FENG PAN et al.
Table 1. Chemical composition of raw kaolin and leached kaolin
Content (wt %)
Sample
SiO2
Al2O3 Fe2O3 TiO2
Na2O
K2O SO3 P2O5
MgO
CaO
LOI
Raw kaolin
45.81
93.94
37.53
4.26
0.21
0.14
0.60
1.26
0.03
–
0.40
–
0.06
0.04
0.07 0.74 0.02
0.09 0.02 0.09
14.53
–
Leached kaolin
als. Kaolinite is a cheap natural clay mineral with CZ-5 (the sample prepared from chemical raw mate-
abundant reserves in some countries and regions, rials), respectively.
which contains aluminum and silicon as main ele-
ments with Si/Al molar ratio ~1. It was inicated that
2.3. Characterization
X-ray diffraction (XRD) patterns were performed
kaolin is a good silica and alumina sources for the syn-
thesis of ZSM-5 [17–19]. However, there are no
reports to clarify the influence of ZSM-5 with differ- using a PANalytical X’pert diffractometer with CuKa
ent crystal sizes on the MTP reaction.
radiation, 40 kV voltage and the 5° ≤ 2θ ≤ 60° scanning
range. Relative crystallinity was determined from the
peak areas between 2θ = 22°–25° using diffraction
patterns of highly crystalline ZSM-5 (SiO2/Al2O3 = 37,
the Catalyst of Nankai University) as a reference. In a
rough calculation from the peak intensity in the XRD
patterns, assuming that the crystallinity of the com-
mercial ZSM-5 was 100%. Accordingly, the relative
crystallinity could be calculated as follows [21]:
In this study, the samples of ZSM-5 with different
crystal sizes were synthesized by using natural kaolin
clay as the only silica and alumina sources and the
obtained ZSM-5 materials were investigated in the
catalytic MTP reaction.
2. EXPERIMENTAL SECTION
2.1. Raw Materials and Pre-Treatment
Relative crystallinity (%) = ΣSp/ΣSr,
(1)
Kaolin clay was obtained from Wuhai (Inner Mon-
golia, China), and the raw material was pre-treated
according to the previous report [18], the composition
of the raw kaolin and leached kaolin obtained precur-
sor is shown in Table 1. Hydrochloric acid, TPAOH
(25%), Sodium silicate (SiO2 60.0 wt %, Na2O 20.0 wt %),
aluminum sulfate Al2(SO4)3 · 18H2O and sodium hydrox-
ide of analytical grade were used without further puri-
fication.
where ΣSp is the peak areas of the products, ΣSr is the
peak areas of the reference sample.
Morphology was inspected using a field-emission
scanning electron microscopy (SEM) (JEM-7001F,
JEOL, Japan).
N2 adsorption–desorption curves were obtained on
AUTOSORB-1 analyzer (Quantachrome Instru-
ments, USA). Prior to measurement, all the samples
were degassed at 573 K for 3 h under vacuum.
Total acidity and acid strength distribution of the
catalyst were determined by calorimetric measure-
ment of differential adsorption of NH3 at 80°C and
2.2. Catalysts Preparation
2.2.1. Synthesis of micro- and nanosized ZSM-5.
The microsized sample was synthesized according to subsequent desorption of the adsorbed NH3 with a
the method earlier described [19], the nanosized
ZSM-5 was synthesized in accordance with our previ-
ously published reports [20].
ramp of 10°C/min up to 600°C (ChemBET 3000,
Quantachrome, USA).
UV Raman spectra were recorded on a UV Raman
spectrograph (Aramis, Jobin-Yvon, Germany) with a
continuous-wave (CW) UV line at 244 nm as the
source.
Coke amounts were determined in a thermogravi-
metric analyzer (TG/DTA 6300, Japan). The samples
were heated from 30 to 800°C in a flowing air with a
heating rate of 10°C/min.
2.2.2. Synthesis of submicron ZSM-5. The precur-
sor was uniformly mixed with NaOH, water and 5.0%
crystal seeds obtained by milling microsized ZSM-5 in
ball mill for 4.0 h (400 rpm). Then the dispersion was
stored at 190°C for 6.0 h in 50 mL stainless steel auto-
clave. After that, the sample was filtered, washed with
deionized water, and dried.
2.2.3. Ammonium exchange. All the obtained sam-
ples were turned into H-form by ion exchange with
1 M NH4NO3 solution at 90°C and then heated at
550°C for 6 h. Four samples were designated as MZ-5
2.4. Methanol Dehydration Reaction
Catalytic reaction tests were conducted in a contin-
(microsized HZSM-5), SZ-5 (HZSM-5 with submi- uous downflow, fixed-bed stainless steel reactor (inter-
cron crystallites), NZ-5 (nanosized HZSM-5) and nal diameter 6 mm). The reaction conditions were as
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SYNTHESIS OF NANO/MICRO SCALE ZSM-5
543
follows: temperature 370°C, WHSV 12 h−1, carrier gas
N2 (25 mL min–1), atmospheric pressure.
The products were analyzed on-line by a gas chro-
matograph (GC-2014, Shimadzu, Japan) equipped
with a flame ionization detector (FID) and a thermal
conductivity detector (TCD). FID with a TG-BOND
capillary column (30 m × 0.32 mm × 10 μm, Thermo,
USA) analyzed the un-reacted methanol, dimethyl
ether and hydrocarbons. The conversion of methanol
was calculated by applying the molar balance between
the inlet and outlet of the reactor as Eq. (1). Selectivity
for the products of interest were expressed as mass per-
centage of each product and calculated according to
the carbon balance between the inlet and the outlet of
the reactor (Eq. (2)).
e
d
c
b
a
5
10
15
20
2θ, deg
25
30
35
40
Nmethanol,i − Nmethanol, j
Fig. 1. XRD patterns of (a) precursor, (b) MZ-5, (c) SZ-5,
(2)
(3)
X methanol
=
×100,
(d) NZ-5, and (e) CZ-5.
Nmethanol,i
3 × NC0 H
Nmethanol,i − Nmethanol,j
schistose with narrow size distribution (axis a × b × c:
(800~1000) × (400~500) × (50~100) nm) and smooth
edged crystals. According to the theory of nucleation
and growth, the nucleation rates exceed the growth
rates when seeding is used [24], which is beneficial for
3
6
,
×100
SC H
=
3
6
where Nmethanol,i was the molar number of methanol
got into the reactor, Nmethanol,j was the molar number of
unreacted methanol, superscript and referred to the production of small crystallites. The sample NZ-5
i
0
the components at the inlet and outlet of reactor.
was composed of nanocrystals in the size range 30 to
40 nm in good agreement with the XRD result. The
CZ-5 obtained from chemical raw materials had the
morphology similar to the MZ-5.
3. RESULTS AND DISCUSSION
3.1. Analysis of Phase and Morphology
Figure 1 shows the XRD patterns of the synthesized
products. The leached metakaolin showed only a flat
and broad peak at 2θ = 23°, indicating that the precur-
sor was X-ray amorphous material. Moreover, two
small peaks at 2θ = 25°–27° due to quarts indicate that
the quarts content was very low. After hydrothermal or
solid-state thermal treatment, all products exhibited
the typical XRD patterns, which correspond to the
MFI structure. The results demonstrated that amor-
phous precursor was converted to a highly crystalline
ZSM-5 phase. Using Eq. (1), the relative crystallini-
ties of MZ-5, SZ-5 and NZ-5 were determined to be
94.2, 96.7 and 95.48%, respectively. The quarts could
not be detected in the final samples due to its very low
content. The decrease in peak intensity and the
increase in line width of NZ-5 were attributed to
decreasing crystal size of the sample [22, 23]. As cal-
culated from the Scherrer equation the crystal size of
NZ-5 was near 43 nm.
3.2. Pore Structure
Figure 3 shows adsorption–desorption isotherms
of nitrogen on ZSM-5 samples prepared with different
crystallite sizes. All samples gave the typical Langmuir
adsorption, showing a steep increase in the curves at
low relative pressures (10–7 < P/P0 < 10–2). This shape
was ascribed to the filling of micropores with N2.
However, obvious difference exists among the iso-
therms registered for these four samples. For CZ-5, a
sharp knee at low relative pressures was followed by
plateau, which is nearly horizontal for adsorption and
desorption branches. As to MZ-5 and SZ-5, an
enhanced uptake and a small hysteresis loop at P/P0 >
0.45 could be observed, which might be caused by the
existence of small amounts of amorphous phase or
quarts. Unlike the above three samples, NZ-5 exhibited
enhanced uptake and obvious hysteresis loop at higher
P/P0, which was attributed to the inter-crystalline voids
formed by the packing of the nanocrystals [25].
Figure 2 shows the SEM images of the kaolin, pre-
cursor and four samples. The original kaolin and pre-
From Table 2, it could be seen that both the micro-
cursor displayed plate-like shape. After hydrothermal pore surface area and external surface area increased
or solid-state thermal treatment, the irregular lamellar with decreasing crystal size. This result is in agreement
precursor transforms to highly crystalline ZSM-5 with with the value earlier reported [26]. Due to the
varying morphology. MZ-5 showed uniform colum- decrease in the crystal size, NZ-5 could provide much
nar structure with microsized crystals (axis a × b × c: larger external surface area for catalytic reaction than
(6~8) × (3~4) × (1~2) μm). In contrast, SZ-5 prepared the other samples, which is beneficial for the dehydra-
in the presence of seed crystals exhibited hexagonal tion reaction.
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544
FENG PAN et al.
2 nm
1 nm
(b)
(a)
(c)
(e)
10 μm
1 μm
(d)
800 mm
(f)
2 nm
Fig. 2. SEM images of (a) kaolin, (b) leached kaolin, (c) MZ-5, (d) SZ-5, (e) NZ-5, and (f) CZ-5.
3.3. Surface Acidity of Catalysts
300
250
200
150
100
50
As shown in Fig. 4, all samples exhibite typical
MZ-5
NH3-TPD spectra with two maximum peaks near 250
SZ-5
and 450°C, which can be ascribed to NH3 removal
from weak and strong acid sites, respectively. Accord-
ing to Table 3, all samples had similar Si/Al ratios.
Moreover, the Na content in the exchanged samples is
NZ-5
CZ-5
negligible, indicating that Na+ ions were extensively
+
exchanged by
ions. Neverthelesser, the concen-
NH4
trations of weak and strong acid were significantly dif-
ferent. Compared with MZ-5, the nanosized NZ-5
sample contains acid sites in a much lower concentra-
tion, which is consistent with the result reported by
Firoozi [5]. Usually, the content of acid sites correlates
with the fraction of the framework aluminum. As
reported by Lopes [27], a decrease in the crystallite
size resulted in a reduction of the content of Si(OH)Al
groups and in increasing amounts of the non-frame-
0
0.2
0.4
0.6
0.8
1.0
Relative pressure, P/P0
Fig. 3. N adsorption–desorption isotherms of ZSM-5
with different crystal sizes.
2
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SYNTHESIS OF NANO/MICRO SCALE ZSM-5
545
Table 2. Parameters of the pore structure of ZSM-5 zeolites
work Al. Therefore, this phenomenon was ascribed to
the different levels of dealumination. Consequently,
the non-framework Al species reacted with the pro-
tonic sites of zeolite, causing a significant reduction of
the protonic acidity. However, SZ-5 had the lowest
content of acid sites among three samples synthesized
from kaolin, which might be associated with an
enhanced concentration of foreign elements (espe-
cially Ti).
In spite of the fact that MZ-5 and CZ-5 had a sim-
ilar crystal size, the peak area of MZ-5 was lower than
that of CZ-5, indicating that the population of weak
and strong acid sites of MZ-5 decreased markedly.
Based on the X-ray fluorescence analysis results
(Table 2), some elements such as Fe, P, Ca, and espe-
cially Ti, are still retained by the precursor as evi-
denced by our previous report [16]. To confirm the
existence of Ti element in the framework, the samples
have been analyzed on a UV Raman spectra (Fig. 5).
In the spectra, two peaks at ca. 521 and 1110 cm–1
could be observed attributable to the asymmetric
stretching vibrations of Ti–O–Si [28]. The results
demonstrated that Ti was incorporated into the zeolite
framework.
prepared with different crystallite sizes
Parameters
MZ-5 SZ-5 NZ-5 CZ-5
Total surface area (m2 g–1) a
Micropore area (m2 g–1) b
337 342 447 354
268 233 306 312
External surface area (m2 g–1) b
Total pore volume (cm3 g–1) c
Micropore volume (cm3 g–1) b
69
0.2
0.1
109 141 42
0.3 0.3 0.2
0.1 0.3 0.1
Note. The date of NZ-5 and CZ-5 were from the previous reports
[20, 21].
a
Determined by BET method at P/P = 0.02–0.07.
0
b
Determined by t-plot method at P/P = 0.4–0.6.
0
c
Calculated at P/P = 0.99.
0
sample was used in reaction for 7.5 h. In contast, the
sample synthesized form kaolin pursued a graduall
increase in selectivity over the same period of time-
on-stream. Among these samples, the selectivity to
propylene shown by NZ-5 gradually increased to 28%
after reaction was performed over a period of 30 h, and
wth further time the high propylene selectivity per-
sisted. NZ-5 provided higher external surface area,
reduced diffusion path length, and made available
more active sites for catalytic reaction. Therefore, the
formation rate of coke on NZ-5 was slower than that
on other samples.
3.4. Catalytic Performance of Sample
The catalytic performance for methanol dehydra-
tion has been investigated on ZSM-5 catalysts pre-
pared with different crystal size. According to Fig. 6a,
the stability of ZSM-5 zeolites was closely related to
their crystal size. It could be observed that the conver-
sion level was almost 100% for all four samples even
though the catalysts could be subject to the carbon
deposition during the first 12 h. As the reaction proc-
ceded further, the methanol conversions on CZ-5 and
MZ-5 dropped. However, the activity of the ZSM-5
with smaller crystallites was higher than that of the
microsized zeolite. The result was in agreement with
the reported data [5], from which it can be inferred
that the nanosized H-ZSM-5 showed higher activity
and stability compared to the H-ZSM-5 sample pre-
pared with microsized crystallites. Thank to the exis-
tence of nanosized crystals, NZ-5 provides a higher
external surface area and shorter diffusion path, which
is beneficial for the diffusion of reactants and prod-
ucts. However, a high content of strong acid sites in
CZ-5 might be a reson why this catalyst demonstrated
a weaker resistance to carbon deposition than MZ-5.
Based on the above results, it could be concluded
that the increase in propylene selectivity was attributed
to the decrease in the crystal size. Because small crys-
tal has shorter channels, the reactants contact with
fewer number of acidic sites and diffuse out more eas-
ily thus effectively suppressing the secondary reaction
of propylene [5, 26]. Moreover, appropriately coke
MZ-5
SZ-5
NZ-5
CZ-5
The effect of crystal size on the selectivity of pro-
pylene is shown in Fig. 6b. According to the results, in
the initial reaction stage, the selectivity topropylene on
SZ-5, NZ-5 and CZ-5 was similar. The propylene
selectivity shown by MZ-5 was lower than that of CZ-5
when the reaction time was confined to 20 h, which
might be attributed to a lower crystallinity of MZ-5.
However, the opposite trend appeared as the reaction
time was extended. It is worth noting that the propyl-
ene selectivity on CZ-5 gradually decreased as the
100
200
300
400
500
600
Temperature, °C
Fig. 4. NH -TPD curves of obtained samples.
3
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546
FENG PAN et al.
Table 3. Chemical composition of ZSM-5 catalysts prepared with different crystallite sizes
Content, wt %
Sample
n
n
Al2O3
SiO2
SiO2
Al2O3
Fe2O3
TiO2
Na2O
MgO
CaO
MZ-5
SZ-5
NZ-5
CZ-5
94.07
93.94
93.35
93.78
4.15
4.26
4.21
4.38
0.05
0.08
0.03
–
1.18
1.36
1.15
–
0.03
–
0.01
0.04
0.02
0.02
0.04
0.06
0.01
–
38.53
37.49
37.69
36.40
0.02
–
deposition was beneficial to the increase of the C2–C4 NZ-5 suppressed the formation of higher hydrocarbons
and promotes the production of propylene. In addition,
the introduction of Fe, Ti, P, and Ca elements into
ZSM-5 catalyst caused a sharp decrease in the popula-
tion of strong acid sites and total surface acidity (Fig. 4).
olefins selectivity. In methanol dehydration reaction,
lower surface acidity is crucial for the formation of
olefins, while higher acidity favors the formation of
aromatics [29]. Therefore, a lower surface acidity of
NZ-5
1110
521
SZ-5
MZ-5
200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400
Raman shift, cm–1
Fig. 5. UV-Raman spectra of the Ti-ZSM-5 samples.
(a)
(b)
40
35
30
25
20
15
10
100
95
90
85
80
75
70
65
60
MZ-5
SZ-5
NZ-5
CZ-5
MZ-5
SZ-5
NZ-5
CZ-5
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
Time on stream, h
Stream on time, h
Fig. 6. (a) Conversion of methanol, (b) selectivity of propylene on ZSM-5 with different crystal size as a function of time-on-stream.
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SYNTHESIS OF NANO/MICRO SCALE ZSM-5
547
sized from kaolin. The crystal size had a significant
effect on the stability of the catalysts and the product
distribution. The ZSM-5 zeolites with smaller crystal-
lites provide higher external surface area, shorter diffu-
sion distances and reduced intensity of surface acidity.
Therefore, nanosized ZSM-5 sample show an
improved deactivation resistance compared to the
microsized and submicronsized specimen. In addi-
tion, due to increased contents of foreign metallic ele-
ments (especially Ti), microsized ZSM-5 synthesized
from kaolin showed better catalytic performance and
stability compared with the sample obtained from
chemical materials. The results described in the article
could provide the information that might be useful to
find new ways to utilize solid waste or natural clay
resources and to synthesize other practically import-
ant zeolites.
100
98
96
94
92
90
88
MZ-5
SZ-5
NZ-5
CZ-5
9.9%
10.6%
III
5.9%
4.5%
I
II
100 200 300 400 500 600 700 800
Temperature, °C
Fig. 7. TG curves of four samples after methanol dehydration
reaction tested at 643 K (m = 0.3 g, WHSV = 12 h ).
–1
cat
ACKNOWLEDGMENT
The authors would like to thank Wuhai Tian-yu
Chemical High-tech Co. Ltd. (China) for the finan-
cial and raw materials assistance.
As a result, ZSM-5 synthesized from kaolin showed bet-
ter catalytic performance and longer lifetime than the
zeolite obtained from chemical materials.
To estimate the extent and rate of carbon deposi-
tion on the catalyst the amount of coke deposition on
the reacted ZSM-5 catalysts was analyzed by a TG
analyzer. According to Fig. 7, three steps could be rec-
ognized on all curves. The first step is related to
desorption of physically adsorbed water from the sam-
ples (I), the second is associated with removal of coke
(II) and the third represent the stage in which the
weight remains constant (III). The weight losses of
carbon deposition on four samples used in the reaction
were ca. 9.9% (MZ-5), 5.9% (SZ-5), 4.5% (NZ-5),
and 10.6% (CZ-5), respectively. The results clearly
showed that the formation rate of coke increased with
increasing crystallite size. For MZ-5 and CZ-5, the
diffusion of the products was hampered due the pres-
ence of long channels. Consequently, secondary reac-
tion of the trapped primary products took place to
produce bulky molecules, which can act as precursors
of coke [30]. By contrast, shorter dimensions of chan-
nels in SZ-5 and NZ-5 favor feasible diffusion of the
products inside the pores and retard the secondary
reaction to form the precursor of coke. Correspond-
ingly, the intensity of coke formation is reduced [30].
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