Modifications in the HZSM-5 zeolite for the selective transformation of ethylene into propylene

Article abstract of DOI:10.1016/j.apcata.2014.04.005

Propylene production in the transformation of ethylene has been studied on HZSM-5 zeolite catalysts of different SiO₂/Al₂O ₃ ratio (30, 80 and 280), doped (1 wt.%) with K or P or treated by mild in situ steaming, in order to attenuate their acidity. The experiments have been carried out in a fixed bed reactor at 500 °C, space time of 3.2(g _catalysth)(mol CH₂)-1 and time on stream up to 15 h. The results highlight the effect catalyst acidity has on ethylene conversion, catalyst stability (deactivation by coke deposition) and propylene yield. The balance between these indices entails the use of a zeolite of moderate acidity subjected to a mild treatment in order to maintain a suitable activity for propylene formation by oligomerization-cracking reactions, and thus minimizing secondary reactions of hydrogen transfer and aromatization. A propylene yield of 37% is obtained for a time on stream of 11.3 h with the HZSM-5 zeolite catalyst of SiO₂/Al₂O₃ = 80 modified by mild in situ steaming treatment.

Full text of DOI:10.1016/j.apcata.2014.04.005

Applied Catalysis A: General 479 (2014) 17–25
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Modifications in the HZSM-5 zeolite for the selective transformation
of ethylene into propylene
∗
Eva Epelde , Andrés T. Aguayo, Martin Olazar, Javier Bilbao, Ana G. Gayubo
Chemical Engineering Department, University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Propylene production in the transformation of ethylene has been studied on HZSM-5 zeolite catalysts of
different SiO2/Al2O3 ratio (30, 80 and 280), doped (1 wt.%) with K or P or treated by mild in situ steaming,
Received 31 January 2014
Received in revised form 27 March 2014
Accepted 1 April 2014
◦
in order to attenuate their acidity. The experiments have been carried out in a fixed bed reactor at 500 C,
−1
space time of 3.2 (gcatalyst h)(molCH2 ) and time on stream up to 15 h. The results highlight the effect
Available online 18 April 2014
catalyst acidity has on ethylene conversion, catalyst stability (deactivation by coke deposition) and pro-
pylene yield. The balance between these indices entails the use of a zeolite of moderate acidity subjected
to a mild treatment in order to maintain a suitable activity for propylene formation by oligomerization-
cracking reactions, and thus minimizing secondary reactions of hydrogen transfer and aromatization. A
propylene yield of 37% is obtained for a time on stream of 11.3 h with the HZSM-5 zeolite catalyst of
SiO2/Al2O3 = 80 modified by mild in situ steaming treatment.
Keywords:
HZSM-5 zeolite
Propylene
Ethylene
Deactivation
Acidity
© 2014 Elsevier B.V. All rights reserved.
1
. Introduction
Light olefin production is essential in the petrochemical indus-
metathesis of butenes and ethylene to form propylene [18–22].
Another interesting route is the oligomerization-cracking, which
has been studied on acid microporous (HZSM-5, HX, HMOR, HY,
H␤, MCM-22, SAPO-34, SAPO-44, SAPO-18) and mesoporous (Al-
SBA-15, Al-MCM-41) materials [23–28]. These works show the
complex relationship between the results and the catalyst prop-
erties (porous structure and acidity): ethylene conversion needs a
high density of sites with high acid strength, but at the same time,
this property favors secondary and hydrogen transfer reactions in
which paraffins, heavy olefins, aromatics and coke are formed. Fur-
thermore, the attenuation of hydrogen transfer reactions by using
catalysts with a severe shape selectivity (as SAPOs) favors the coke
deposition, which is retained in the cages that are characteristic of
the porous structure of these catalysts.
This work aims to explore the possibilities of HZSM-5 zeolite
in the transformation of ethylene by intensifying propylene pro-
duction. For that purpose the properties of HZSM-5 have been
modified by doping with 1 wt.% of K or P or by mild in situ steam-
ing treatment. In addition, special attention has been paid to the
deactivation of the catalysts by coke deposition, thus determining
the effect the modifications of HZSM-5 zeolite have on the stabil-
ity. It is well-known the good performance of HZSM-5 zeolite in
the processes for light olefin production, due to its high activity
and selectivity, with a limited deactivation by coke as its moderate
acidity and its three-dimensional porous structure, without cages
in the intersections of the pores, facilitates the diffusion of the reac-
tion medium compounds, thus minimizing the retention of coke
precursors [29,30]. The treatments studied in this work have also
try and currently it principally takes place by steam cracking units,
fluid catalytic cracking (FCC), MTO (methanol to olefins) process
and propane dehydrogenation [1]. However, in these processes
the propylene selectivity obtained is not enough for satisfying its
increasing demand for the production of polypropylene, propylene
oxide and acrylonytrile [2]. New developing routes are enhanced
as: (i) ethylene metathesis [3,4]; (ii) higher olefin catalytic crack-
ing [5]; (iii) processes that use natural gas or coal as raw material,
via methanol (MTP process) [6–8] or dimethyl ether (DTP process)
[
9,10], and; (iv) light olefin interconversion [11–14]. Furthermore,
there is an increasing interest in the development of routes for
green propylene” production using raw materials derived from
biomass including bio-ethanol [15], bio-oil [16] and vegetable oils
17].
The direct conversion of ethylene into propylene has caught a
“
[
great attention in order to adjust the production of both olefins to
the market requirements, since ethylene is the main olefin in the
aforementioned processes. The metathesis route has been studied
over metallic catalysts (Ni, Mo, W) supported on different meso-
porous materials (␥-Al O , MCM-41). It takes place by following
two steps: (i) the dimerization of ethylene to butenes and, (ii) the
2
3
∗ Corresponding author. Tel.: +34 946 015 361; fax: +34 946 013 500.
E-mail address: eva.epelde@ehu.es (E. Epelde).
http://dx.doi.org/10.1016/j.apcata.2014.04.005
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

1
8
E. Epelde et al. / Applied Catalysis A: General 479 (2014) 17–25
1
.6
.2
Nomenclature
1
- HZ-80
4
2
- HZ-80-ST
B/L
Fi
Brönsted/Lewis ratio
molar flowrate of i lump in the product stream ((mol
1
3- 1P/HZ-80
4- 1K/HZ-80
−1
of CH ) h
)
2
F0 and F ethylene molar flowrate in the feed and in the outlet
−
1
0.8
0.4
0.0
stream ((mol of CH ) h
)
2
PE
SBET
Si
ethylene partial pressure (bar)
BET surface area (m g
selectivity of i component
2
−1
)
3
1
t
time (min)
2
0
V and Vp volume of N adsorbed at P/P = 0.2 and pore volume
2
3
−1
)
(
cm g
W
X
Yi
εL, εB
catalyst mass (g)
ethylene conversion
150
200
250
300
T (ºC)
350
400
yield of i component
molar extinction coefficients (cm ␮mol−1)
Fig. 1. t-BA-TPD profiles for parent and modified HZ-80 zeolites.
The surface area and porous structure have been determined
by N2 adsorption–desorption (Micromeritics ASAP 2010). The
total acidity and acid strength distribution of the catalysts have
been determined by monitoring the adsorption–desorption of
tert-butylamine (t-BA), by combining the techniques of thermo-
gravimetric analysis and differential scanning calorimetry during
shown good results in the literature in order to modify the micro-
porous structure and to attenuate the acid strength of the zeolite
[
31–33] as well as the coke formation in different processes for
olefin production [34,35].
◦
the adsorption of the base at 150 C. The total acidity is quan-
2
. Experimental
◦
tified as the amount of the base chemisorbed at 150 C [38], in
−
1
mmolt-BA (gzeolite
)
, whereas the acid strength is defined as the
2.1. Catalysts preparation
heat released during the adsorption of the base by mass unit of
−
1
adsorbed base, kJ (mmolt-BA
)
.
The commercial HZSM-5 zeolites (denoted HZ-X, where X indi-
◦
Catalyst saturation at 150 C is followed by temperature pro-
grammed desorption (TPD) (at 5 C min
cates the SiO /Al O molar ratio (30, 80, 280)) were supplied in
◦
−1
◦
2
2
3
up to 550 C) using
ammonium form by Zeolyst International and were calcined at
a Setaram TG–DSC calorimeter connected on-line with a Ther-
mostar mass spectrometer from Balzers Instruments [38]. The TPD
of the amines is based on the formation of alkylammonium ions
◦
5
70 C in order to obtain the acid form. The addition of 1 wt.%
of K or P into the zeolites was carried out by incipient wetness
◦
impregnation, at 70 C under vacuum, with solutions of suitable
(from adsorbed alkyl amines protonated by Brönsted sites), which
concentrations of KOH (85% purity, Panreac) or H PO (85% purity,
3
4
undergo decomposition (cracking) to give ammonia and olefins
in a well-defined temperature range via a reaction similar to the
Merck), respectively, in a rotavapor BÜCHI R-114. The modified
zeolites have been denoted 1K/HZ-X or 1P/HZ-X.
◦
Hofmann-elimination reaction [39]. Above 150 C (and thus in the
Mild in situ steaming treatment is a dealumination method that
uses the structural water contained in the zeolite [33]. The zeolites
TPD temperature range), t-BA is cracked on the acid sites releasing
butene, whose signal (m/e = 56) is monitored by mass spectrom-
eter, thereby identifying sites of enough acidity for cracking t-BA
◦
−1
◦
were preheated with a temperature ramp of 4 C min from 25 C
◦
to a pre-fixed temperature (400 C), which was maintained for 10 h.
[
40]. Moreover, since strong acid sites are active for cracking at
The modified zeolites have been denoted HZ-X-ST.
lower temperature, the temperature of the peaks for the formation
of butene in the TPD indicates the acid strength of the sites. Thus,
the lower the temperature of the peak in the TPD, the higher the acid
strength of the sites, which is the opposite effect to that observed in
The catalysts have been prepared by pelletizing each HZSM-5
zeolite (25 wt.%) with bentonite (Exaloid, 30 wt.%) as binder and ␣-
◦
alumina (Martinswek, 45 wt.%, calcined at 1000 C) as inert charge
(
filler). The catalyst particles were obtained by wet extrusion and
the TPD analysis of commonly used NH . Besides, a small fraction
3
they were sieved to a particle diameter between 0.15 and 0.3 mm.
of t-BA is desorbed without cracking. This desorption of t-BA (sig-
nal at m/e = 58) corresponds to the base physisorbed on weak acid
sites, which are inactive for the cracking of t-BA, and consequently
also for the reactions involved in the transformation of ethylene.
Moreover, the interest of using this base is noteworthy, since
t-BA allows a greater sensitivity in the thermo-gravimetric mea-
surement of the base (due to its higher molecular weight) than NH3.
This sensitivity is required for determining rigorously the differen-
tial adsorption heat of the base, especially for catalysts of moderate
acid strength as some of the samples studied in this work.
◦
The particles were dried in an oven at 110 C for 24 h and then
◦
calcined at 570 C for 2 h. The pelletization of the zeolite provides
the catalyst particle with a matrix with mesopores and macropores,
which contributes to reducing deactivation by coke deposition (as
the blockage at the mouths of the zeolite micropores is minimized)
and to increasing the hydrothermal stability in the regeneration
step by coke combustion [36,37]. Furthermore, ␣-alumina avoids
the partial blockage of zeolite micropores, which is produced by
using only bentonite in the pelletization. Casta n˜ o et al. have studied
the role of different binders and fillers in the pelletization of HZSM-
As an example of the acidity measurements (results summarized
in Table 1), Fig. 1 shows the t-BA-TPD profiles of the parent and
modified HZ-80 zeolites. The unmodified HZ-80 zeolite shows two
5
zeolite [37].
◦
2.2. Catalysts characterization
main peaks: one at low temperature (190 C) related to strong acid
◦
sites and the second peak identified at higher temperature (270 C)
The metallic content of the samples has been determined by ICP-
AES in a Jobin Yvon (ACTIVA Model) spectrometer and the results
have ratified the nominal contents of K and P.
corresponding to weak acid sites, that require higher temperature
for cracking t-BA. The doping with P of HZ-80 zeolite entails a shift
of the first peak toward higher temperatures, which correspond

E. Epelde et al. / Applied Catalysis A: General 479 (2014) 17–25
19
Table 1
Physical and acid properties of the fresh zeolites.
SBET (m2
g
−1
)
Va (cm3
g
−1
)
Vpb (cm3
g
−1
)
Total acidity (mmolt-BA) g
−1
Average acid strength
◦
B/L ratio at 150 C
Zeolite
−
1
(
kJ molt-BA
)
HZ-30
HZ-30-ST
448
447
434
413
137
136
132
126
0.151
0.150
0.147
0.151
0.67
0.57
0.59
0.58
150
153
110
145
0.70
0.58
0.20
0.57
1
1
K/HZ-30
P/HZ-30
HZ-80
HZ-80-ST
556
495
456
464
168
150
140
142
0.110
0.121
0.100
0.109
0.42
0.36
0.38
0.41
145
146
85
0.32
0.29
0.05
0.27
1
1
K/HZ-80
P/HZ-80
140
HZ-280
HZ-280-ST
512
480
415
423
144
132
121
126
0.101
0.062
0.068
0.089
0.15
0.15
0.13
0.15
120
120
75
–
–
–
–
1
1
K/HZ-280
P/HZ-280
120
a
Volume of N2 adsorbed at P/P0 = 0.2.
Pore volume, 17 < dp < 3000 A.
b
˚
to a lower acid strength of the strong acid sites. This effect is also
observed for the weak acid sites, whose peak is lower and with
a maximum slightly shifted toward higher temperature. The mild
in situ steaming treated zeolite, HZ-80-ST, shows a lower amount
of strong acid sites. However, a slight deviation of the first peak
80
700
600
500
400
300
200
1- HZ-80
2
3
- HZ-80-ST
- 1P/HZ-80
6
4
2
0
0
0
0
4- 1K/HZ-80
◦
toward lower temperature (187 C) is observed, which indicates a
slight increase in acid strength of these sites. The area of the peak
corresponding to weak acid sites is similar to that for the parent
zeolite, but this maximum shifts toward higher temperature, which
indicates that the acid strength of the weak sites has been attenu-
ated. This evolution of acid strength is more significant when K is
added into the zeolite, and consequently, for 1K/HZ-80 a sole peak
is observed in the TPD profile that corresponds to sites of moderate
acid strength, where the presence of weak acid sites is insignificant.
The Brönsted/Lewis (B/L) acid site ratio has been determined in
a Specac catalytic chamber connected on-line with a Nicolet 6700
FTIR spectrometer, from the ratio between the intensity of FTIR
1
2
3
4
0
20
40
60
t (min)
Fig. 2. TPO profiles for the coke deposited on the parent and modified HZ-80
−
1
◦
adsorption bands at 1545 and 1450 cm of adsorbed pyridine at
catalysts used in the transformation of ethylene. Conditions: 500 C, W/FE0 =
−
1
◦
3.2 (gcatalyst h)(molCH2
)
, PE = 1.35 bar, TOS = 5 h.
1
50 C, and taking into account the molar extinction coefficients
of both adsorption bands proposed by Emeis (εB = 1.67 cm ␮mol
−
1
−
1
◦
◦
and εL = 2.22 cm ␮mol ) [41].
temperature (475 C), and coke II, which peaks at 555 C. Coke I is
related to more hydrogenated species, and is presumably located
in the mesopores of the catalysts (externally to the crystals of the
zeolite, on the catalyst matrix composed of bentonite and alumina),
whereas coke II is related to more condensed species (higher C/H
ratio) and is preferably internally located in the micropores of the
zeolite [42,43]. The presence of the external coke (coke I) involves
a high coke formation rate, which is not reached for the modified
catalysts. Consequently, it should also be noted that the external
coke (coke I) is almost absent in the modified catalysts, which is
in agreement with the fact that the coke is originally formed in
the micropores (coke II) and then it is swept toward the outside of
the micropores, in order to be deposited in the mesoporous matrix
(coke I) [43].
Although this work focuses on the study of the effect HZSM-5
modifications have on its properties and particularly on its acidity,
structural alterations of the zeolite may have an influence on these
properties. XRD analysis (PANalytical Xpert PRO difractometer) has
proven that the method of doping with K or P does not alter HZSM-5
crystalline structure significantly, as only slight deviations in unit
27
29
cell parameters were observed. Furthermore, Al and Si NMR
results (Bruker 400 WB PLUS (9.40 T) spectrometer) revealed that
P modification gives way to the transformation of framework TAl
into OAl, whereas KOH treatment caused a severe desilication of the
zeolite (with changes in Si nucleus from Q4 to Q coordination).
The coke content on the catalyst was determined by
temperature-programmed oxidation (TPO) in a TGA Q5000 ther-
3
mobalance (TA Instruments). After stabilization of the sample at
◦
2
50 C for 30 min in a He stream, the oxidant mixture (25% O2 in
2.3. Reaction equipment and product analysis
He) was introduced and the combustion took place by following a
◦
−1
◦
heating ramp of 8 C min up to 600 C, and this temperature was
maintained for 1 h to complete coke combustion.
The runs have been carried out under atmospheric pressure in
an automated reaction equipment (Microactivity Reference of PID
Eng&Tech., Madrid, Spain) [38]. The reactor is made of 316 stain-
less steel with an internal diameter of 9 mm and 10 cm of effective
length. It is located inside a ceramic covered stainless steel cylin-
drical chamber, which is heated by an electric resistance and can
As example, Fig. 2 shows the TPO profiles corresponding to
the combustion of the coke deposited on the parent and modi-
fied HZ-80 zeolite catalysts after 5 h time on stream. Overall, the
modifications of the parent zeolite by the treatments studied give
way to a lower deposition of coke due to the lower acidity and acid
strength of the sites, with 1K/HZ-80 catalyst being the one with
the lower coke content. In the profile for the parent zeolite (HZ-
◦
operate up to 100 atm and 700 C with a catalyst mass of up to 5 g.
The bed consists of a mixture of catalyst and inert solid (carborun-
dum with an average particle diameter of 0.105 mm) to ensure bed
isothermality and attain sufficient height under low space time
8
0) two types of coke are identified: coke I, which burns at low

2
0
E. Epelde et al. / Applied Catalysis A: General 479 (2014) 17–25
conditions. The temperature is controlled by a digital TTM-125
Series controller and measured by a thermocouple (K type) situated
in the catalyst bed. There are two more temperature controllers:
one for the furnace chamber and the other for the transfer line
between the reactor and the micro-GC. At the end of each run,
temperature is kept constant and catalyst sweeping is carried out
with a helium flow of 30 cm min for 30 min, which stabilizes and
homogenizes the coke deposited on the catalyst for further anal-
ysis. The operating variables are controlled by bespoke software
Br
Lw
a
1
K/HZ-30
1
P/HZ-30
3
−1
HZ-30-ST
HZ-30
(
Process@ from PID Eng&Tech, Madrid, Spain).
A fraction of the reaction products (diluted in a He stream of
3
−1
1
7 cm min ) is continuously sent to a gas chromatograph (Agi-
1
700
1600
1500
Wavenumber (cm )
1400
1300
lent MicroGC 3000A) for its analysis, and the remaining stream
of reaction products passes through a Peltier cell at 0 C. The
-1
◦
amount of liquid condensate is controlled by a level sensor and the
non-condensable gas flow is vented. The micro-GC (with Soprane
software) is provided with 4 analytical modules and the follow-
ing columns: a molecular sieve (MS-5A) (10 m) for analysis of CH4;
Porapak Q (PPQ) (8 m), for analysis of C –C light olefins; Alumina
Br
Lw
b
1K/HZ-80
2
3
(
10 m), for analysis of light hydrocarbons (C –C5 light paraffins and
1P/HZ-80
HZ-80-ST
HZ-80
2
olefins); and OV-1, for analysis of C5–C10 fraction and aromatics
BTX). The product stream is analyzed every 4 min. The quantifi-
(
cation and identification of the compounds was carried out based
on calibration standards of known concentration. The balance of
atoms (C, H) is closed in all runs above 99.5%.
1
700
1600
1500
1400
1300
-1
3
. Results and discussion
Wavenumber (cm )
◦
Fig. 3. FTIR spectra of pyridine adsorbed at 150 C. Graph a, parent and modified
3.1. Effect of zeolite modifications on catalysts properties
HZ-30 zeolites. Graph b, parent and modified HZ-80 zeolites.
Table 1 summarizes the physical properties of the different zeo-
lites (prior to pelletization). The doping with K or P alters the porous
structure by reducing the BET surface area and micropore vol-
ume, which could be attributed to the partial blockage of the pores
the dehydroxilation is significant. However, this temperature is
required for the equilibration of the acid structure in order to attain
reproducible performance in successive reaction–regeneration
[
32,44,45]. Mild in situ steaming treatment has a similar effect due
◦
to the zeolite dealumination [33]. Furthermore, pelletizing the zeo-
lite with bentonite and alumina gives way to a matrix with meso-
and macropores [46].
cycles, by carrying out the coke combustion at 550 C [49]. B/L ratio
decreases with an increase in SiO2/Al2O3 ratio, which is character-
istic of HZSM-5 zeolite due to the decrease in Brönsted sites and an
increase in Lewis sites [45].
The results in Table 1 show that total acidity and average
acid strength (determined by differential adsorption of t-BA at
The doping with 1 wt.% K entails a decrease in the amount of
Brönsted acid sites, the absorbance signal being almost negligible
for 1K/HZ-80 zeolite. This effect has also been reported by other
authors [45,50]. It should be noted that the FTIR spectra for the
◦
1
0
0
50 C) notably decrease with an increase in SiO /Al O ratio (from
2
2
3
−1
1
−1
.67 mmolt-BA (gzeolite
.15 mmolt-BA (gzeolite
)
)
and 150 kJ molt-BA for HZ-30 zeolite to
and 120 kJ molt-BA for HZ-280 zeolite).
−
−1
−
1
Moreover, the doping of HZSM-5 zeolite by incorporating 1 wt.% K
or P is effective for reducing total acidity and homogenizing acid
strength. These results have also been reported in the literature
zeolites modified by K show a slight shift of the band at 1455 cm
(Lewis sites) toward lower wave number, which indicates that
these sites are weaker than those corresponding to the parent zeo-
lite [50,51]. From these results it is concluded that Brönsted acid
sites are highly affected by the doping with KOH and, as a conse-
quence of its partial transformation into Lewis sites, the amount of
the latter increases.
[
32,44,45,47]. Mild in situ steaming treatment also decreases total
acidity, but in the case of HZ-30-ST and HZ-80-ST zeolites a slight
increase of the acid strength is observed, which is explained by
the formation of dislodged Al in the extra framework [48]. Over-
all, among the treatments studied, K incorporation is the most
severe, thus reducing acid strength from values of 150–112 to
The zeolites doped with P also show an attenuation of the FTIR
bands corresponding to both types of acid sites [32,44,52], espe-
cially of the Brönsted acid sites. Mild in situ steaming treatment
−1
1
07–75 kJ molt-BA . Furthermore, pelletization of the zeolites does
◦
not contribute to modifying their acidity, as the acidity of ben-
tonite and alumina is insignificant. Consequently, the total acidity
of the catalysts is a quarter of the corresponding zeolites, with acid
strength being similar.
at 400 C results in a decrease of the amount of Brönsted sites,
although the average acid strength is slightly affected.
The values of Brönsted/Lewis ratio (Table 1) have been deter-
mined from the FTIR spectra of pyridine adsorbed at 150 C. In
3.2. Catalyst performance at zero time on stream
◦
Fig. 3 the spectra corresponding to the parent and modified HZ-
In order to compare the kinetic performance of the catalysts,
the following reaction indices have been studied: (i) ethylene con-
version (X); (ii) yield (Y ), and; (iii) selectivity (S ) of each lump
3
0 (Fig. 3a) and HZ-80 zeolites (Fig. 3b) are shown. The low total
acidity of HZ-280 (SiO /Al O = 280) zeolite has hindered obtaining
2
2
3
i
i
reproducible results.
The values of Brönsted/Lewis ratio (Table 1) are in general
low due to the high calcination temperature (570 C), for which
of products. The following lumps of products have been consid-
ered: CH , C H , C –C paraffins, C H , C H10, C5+ aliphatics, and
aromatics (BTX).
4
3
6
2
3
4
8
4
◦

E. Epelde et al. / Applied Catalysis A: General 479 (2014) 17–25
21
Ethylene conversion is defined as:
100
F − F
0
X =
× 100
(1)
a
F
0
8
0
C
c
(wt.%)
where F and F are the molar flow rate of ethylene in the feed and in
0
the reactor outlet stream, respectively (expressed in CH equivalent
units).
The yield of each lump of product is calculated considering the
molar flow rate of the i lump in the product stream (expressed in
CH2 equivalent units):
HZ-30
5.6
2.4
3.2
2.9
2
60
40
1K/HZ-30
1P/HZ-30
HZ-30-ST
Fi
Yi =
× 100
(2)
F0
2
0
0
The selectivity is defined as:
Fi
Si =
× 100
(3)
F0 − F
0
50
100
150
200
250
300
◦
The operating conditions have been: pressure, 1.5 bar; 500 C;
feed, 35 cm min of ethylene diluted in He (90% of ethylene); cat-
t (min)
3
−1
alyst mass, 0.55 g; space time, 3.2 (g of catalyst) h (mol of CH in
50
40
30
20
10
0
2
−
1
the feed) ; time on stream, up to 15 h. Blank runs (without cata-
lyst) were carried out to confirm that thermal cracking of ethylene
is insignificant. It has also been proven that bentonite and alumina
have an insignificant activity in the transformation of ethylene. The
runs have been repeated three times to assess experimental repro-
ducibility and the results differ by less than 1% from the average
value.
Table 2 shows the results at zero time on stream (fresh catalysts)
of ethylene conversion, light olefin (propylene and butenes) yield
and the selectivity of the different lumps of products.
b
Ethylene conversion decreases as the SiO /Al O ratio is
2
2
3
increased, due to the decrease in total acidity and acid strength
of the zeolite. However, propylene selectivity increases, reach-
ing a value of ∼50% for HZ-280 catalyst, which is explained
by the decrease in acidity and the acid strength of the sites
that hinder oligomerization, hydrogen transfer and aromatization
reactions, which give way to paraffin and aromatic formation.
As a consequence of the opposite effect of the SiO /A O ratio
0
50
100
150
200
250
300
t (min)
2
2
3
on ethylene conversion and propylene selectivity, the maximum
propylene yield for the parent zeolites (20%) corresponds to an
intermediate SiO /A O ratio (HZ-80 catalyst). From a point of
Fig. 4. Effect of the modifications of HZ-30 zeolite on the evolution with time
◦
on stream of ethylene conversion (a) and propylene yield (b). Conditions: 500 C,
−
1
W/FE0 = 3.2 (gcatalyst h)(molCH2
)
, PE = 1.35 bar.
2
2
3
view of scaling-up, the drawback of the low ethylene conversion
obtained with HZ-280 catalyst could be solved by recycling the
byproduct stream. This stream is mainly composed of pentenes and
butenes that are susceptible of cracking to produce light olefins.
These aforementioned results of the effect of SiO /A O ratio
in [26] for the transformation of ethylene on H-Na-HZSM-5 zeolites
with different protonation levels, which evidence the essential role
of the Brönsted acid sites in this reaction.
The doping with 1 wt.% of P gives way to a decrease in ethylene
conversion and a slight increase in propylene selectivity (Table 2).
This effect, similar to that observed by Lin et al. [26] for different
P/Al ratios (between 0.8 and 1.5), is lower than that corresponding
to K modification, which is explained by a lower modification of
acidity (Table 1).
The catalysts treated by mild in situ steaming presumably do not
provide interesting results in order to intensify propylene produc-
tion (Table 2). Although modified HZ-30 and HZ-80 show a higher
ethylene conversion in comparison to their parent zeolites, the
increase in acid strength after this treatment (Table 1) results in
a decrease in propylene selectivity at zero time on stream. How-
ever, as it is shown in next Section, these catalysts show a high
propylene yield when they are partially deactivated.
2
2
3
of HZSM-5 zeolite in the transformation of ethylene are consistent
◦
with those obtained by Lin et al. [26] at 450 C, even though the
effect on propylene selectivity is lower than the obtained in this
work. Song et al. showed a similar qualitative effect on the trans-
formation of ethanol to propylene on HZSM-5 zeolites of different
◦
SiO /Al O ratio [53]. In this reaction, above 400 C the dehydration
2
2
3
of ethanol to ethylene on HZSM-5 zeolite is almost immediate and
it takes place at the reactor entrance [54], thus, product distribution
is controlled by ethylene transformation.
Furthermore, the results in Table 2 show the notable effect the
modifications of HZSM-5 zeolite have on the kinetic performance.
It is observed that the doping with 1 wt.% of K has a more relevant
effect than the other treatments studied, as the physicochemical
properties of the zeolite are greatly modified, especially the acidity
(Table 1). Thus, ethylene conversion is 32.6% for HZ-280 catalyst, in
3.3. Catalysts stability
comparison to the 2.4% for 1K/HZ-280 catalyst, whereas propylene
selectivity increases from 50.5% to 70.5%, respectively. It should
be mentioned that the doping with K shows a minor effect on the
reaction indices for the zeolites with low SiO /Al O ratio. These
3.3.1. Effect of zeolite modifications
Figs. 4–6 show the evolution with time on stream of ethylene
conversion (graphs a) and propylene yield (graphs b), for the cata-
lysts whose kinetic behavior at zero time on stream was analyzed
2
2
3
results of K addition are consistent with those previously reported

2
2
E. Epelde et al. / Applied Catalysis A: General 479 (2014) 17–25
Table 2
Effect of the HZSM-5 zeolite modifications on the values at zero time on stream for conversion, product yield and selectivity. Operating conditions: 500 C, W/FE0
◦
=
−1
3.2 (gcatalyst h)(molCH2
)
, PE = 1.35 bar.
Catalyst
X (%)
YC3
H
(%)
YC4
H
(%)
Selectivity (%)
CH4
6
8
C3H6
C4H8
C2–C3
C4H10
C5+
BTX
HZ-30
97.3
96.9
97.0
97.9
3.7
5.1
4.0
3.3
1.0
2.0
0.7
1.9
3
3.8
5.3
4.1
3.4
1
46.9
43.5
48
13.1
19.4
13.7
12.2
8.2
24.0
18.1
22.1
28.6
1
1
K/HZ-30
P/HZ-30
2.4
3.4
3.9
2.1
0.7
1.9
9.2
8.0
1.5
HZ-30-ST
48.5
HZ-80
90.0
87.3
89.8
95.1
20.0
23.8
20.5
16.0
7.9
13.0
8.3
0.8
0.4
1.1
1.5
22.2
27.3
23.5
16.8
8.8
14.9
9.4
22.9
14.7
24.5
27.2
12.8
15.5
16.4
12.1
22.1
23.1
18.3
20.0
10.4
4.1
7.5
1
1
K/HZ-80
P/HZ-80
HZ-80-ST
9.6
10.1
12.3
HZ-280
32.6
2.4
31.7
20.4
16.5
1.7
15.3
8.9
6.1
0.02
7.4
5.7
0
0
0
0
50.5
70.5
48.3
43.7
18.6
0.7
23.3
28.0
2.4
0
1.7
0.84
1.6
0
1.3
1.0
25.5
28.8
24.8
24.9
1.4
0
0.6
1
1
K/HZ-280
P/HZ-280
HZ-280-ST
1.56
in the previous Section. The results for the catalysts prepared based
on a specific SiO /Al O ratio are grouped in each figure. The coke
contents deposited on the catalysts after 5 h time on stream are also
summarized in these figures.
catalyst stability. For HZ-30 catalyst the decrease in conversion
is not significant at low values of time on stream (Fig. 4a), as
it is the most active catalyst with an initial conversion close to
the thermodynamic equilibrium. Based on the evolution of the
results along 5 h time on stream, it is observed that the deacti-
vation is more pronounced as the SiO /Al O ratio is increased
2
2
3
The comparison of the results for the parent catalysts (prior
to modification) shows the relevant effect of SiO /Al O ratio on
2
2
3
2
2
3
1
00
1
00
a
C
c
(wt.%)
8
6
4
2
0
0
0
0
0
8
6
4
2
0
0
0
0
0
a
HZ-280
K/HZ-280
1P/HZ-280
HZ-280-ST
1.3
1.1
1.0
1.9
1
X
C
c
(wt.%)
HZ-80
4.7
1.3
1.6
1.9
1
1
K/HZ-80
P/HZ-80
HZ-80-ST
0
50
100
150
200
250
300
0
50
100
150
200
250
300
t (min)
t (min)
5
0
0
0
0
0
0
50
40
30
20
b
b
4
3
2
1
1
0
0
0
50
100
150
200
250
300
0
50
100
150
200
250
300
t (min)
t (min)
Fig. 5. Effect of the modifications of HZ-80 zeolite on the evolution with time
Fig. 6. Effect of the modifications of HZ-280 zeolite on the evolution with time
◦
◦
on stream of ethylene conversion (a) and propylene yield (b). Conditions: 500 C,
on stream of ethylene conversion (a) and propylene yield (b). Conditions: 500 C,
−
1
−1
W/FE0 = 3.2 (gcatalyst h)(molCH2
)
, PE = 1.35 bar.
W/FE0 = 3.2 (gcatalyst h)(molCH2
)
, PE = 1.35 bar.

E. Epelde et al. / Applied Catalysis A: General 479 (2014) 17–25
23
Oligomerization-
cracking
Hydrogen transfer,
aromatization
1
00
Ethylene
Propylene
Butenes
Paraffins
Aromatics
Coke
Ethylene
a
reactions
8
0
Fig. 7. Reactions involved in ethylene transformation over HZSM-5 catalyst.
HZ-30
P/HZ-80
HZ-80-ST
(
HZ-30 < HZ-80 < HZ-280) and it has as a consequence: (i) the pro-
60
40
20
0
1
gressive decrease in ethylene conversion, observed for all the
catalysts (Figs. 4a–6a), and; (ii) the evolution of propylene yield,
which increases with time on stream for HZ-30 (Fig. 4b) and HZ-80
(Fig. 5b) catalysts and decreases for HZ-280 catalyst (Fig. 6b).
It should be noted that the evolution of propylene yield with
time on stream is explained by its role as intermediate compound
in the kinetic scheme for ethylene transformation (Fig. 7), which
includes steps in series. Thus, when the catalysts have an exces-
sive acidity for the first step and the reaction is controlled by the
second step, as it is the case for HZ-30 and HZ-80 catalysts, coke
deposition affects firstly to the velocity of hydrogen transfer and
aromatization reactions, which require a high density of strong
acid sites, and therefore, the yield of intermediate compounds (light
olefins) increases. Subsequently, the deactivation will affect to the
oligomerization-cracking reactions that are responsible for light
olefin formation (first step in Fig. 7), which require a lower acid
strength, which justifies the maximum in the evolution of propyl-
ene yield with time on stream that is observed in most of the results
shown in Figs. 4b–6b. Nevertheless, when the catalysts do not have
enough acidity for the first step in Fig. 7, coke deposition affects to
the velocity of this step from zero time on stream, thus reducing
propylene yield.
0
100 200 300 400 500 600 700 800 900
t (min)
5
4
3
2
1
0
b
0
0
0
0
0
Considering that coke formation on HZSM-5 zeolite depends on
the acidity (mainly due to the presence of strong acid sites) and on
the coke precursors’ concentration in the reaction medium (eth-
ylene being a well-known precursor) [55,56], the results of coke
contents (shown in Figs. 4–6) for the parent zeolite catalysts: HZ-30
(
5.6 wt.%) > HZ-80 (4.7 wt.%) > HZ-280 (1.3 wt.%) prove the funda-
0
100 200 300 400 500 600 700 800 900
mental role of acidity on coke formation. Furthermore, the decrease
in ethylene conversion and the evolution of propylene yield with
time on stream are more pronounced when the initial acidity of
the zeolite is low, despite of the lower coke deposition, because
due to the lower amount and acid strength of the sites, they are
more affected by the presence of coke.
t (min)
Fig. 8. Evolution with time on stream of ethylene conversion (a) and propylene
◦
yield (b) for HZ-30, 1P/HZ-80 and HZ-80-ST catalysts. Conditions: 500 C, W/FE0
=
−1
3
.2 (gcatalyst h)(molCH2
)
, PE = 1.35 bar.
The results in Figs. 4–6 for the catalysts prepared with modified
zeolites show similar trends to those aforementioned for their par-
ent zeolites concerning the effect of acidity on the evolution with
time on stream of ethylene conversion, propylene yield and coke
deposition. Furthermore, zeolite modifications have an important
effect on the results of conversion, propylene yield and stability,
which are dependent on the acidity and acid strength of the cata-
lysts.
The catalysts modified by P (1P/HZ-80) and by mild in situ
steaming (HZ-80-ST) have a suitable acid structure, in order to
on the one hand, decrease coke deposition (1.6 wt.% and 1.9 wt,%,
respectively), and on the other hand, avoid the inactivity of the
acid sites by coke deposition. Therefore, these catalysts are very
stable (Fig. 5a) and propylene yield increases with time on stream
Fig. 5b). The increase in the stability of HZSM-5 zeolite after
these treatments has been previously reported for other processes
44,52,57,58].
(
The modified HZ-30 zeolite catalysts (Fig. 4) give way to a more
pronounced decrease in ethylene conversion with time on stream
[
As a consequence of the aforementioned relevant effect of
(Fig. 4a), although propylene yield reaches a maximum, which is
SiO /Al O ratio of HZSM-5 zeolite, the attenuation of the acidity of
2
7% for 1K/HZ-30 catalyst and corresponds to 250 min time on
2
2
3
modified HZ-280 parent zeolite, of low activity due to its low acid-
ity, only causes a decrease in initial conversion and catalyst stability
Fig. 6a) and the propylene yield is lower for the modified zeolites
than that for the parent zeolite, which decreases steadily with time
on stream (Fig. 6b).
stream (Fig. 4b).
Mild in situ steaming of HZ-80 zeolite slightly increases the
initial conversion and catalyst stability (Fig. 5a), with a lower pro-
pylene yield, which increases steadily along 5 h time on stream
(
(
Fig. 5b). The doping with P enhances catalyst stability as well as
propylene yield, which increases steadily with time on stream to
0% at 5 h. Furthermore, the doping with K decreases catalyst sta-
3
3.3.2. Suitable catalyst and operating strategy
bility, with a maximum propylene selectivity of 26.5% at 50 min
time on stream. The doping with K is a very severe treatment that
excessively attenuates catalyst acidity in order to obtain a suitable
balance between activity–stability–propylene yield. Although the
coke content is low (1.3 wt.%), the acid sites are very sensitive to
this coke.
Among the results explained in Section 3.3.1, it should be noted
that the maximum of propylene yield evolution with time on
stream for some catalysts, allows considering the use of a fluidized
bed reactor with catalyst circulation (partially deactivated catalyst
in the outlet and regenerated catalyst in the inlet) as a suitable oper-
ating strategy, which should be designed with an average residence

2
4
E. Epelde et al. / Applied Catalysis A: General 479 (2014) 17–25
time equal to the time on stream where the maximum propylene
yield is achieved. This strategy has already been industrially imple-
mented in the MTO (methanol to olefins) process [59] and it has
been recently proposed for the catalytic pyrolysis of lignocelullosic
biomass [60].
Acknowledgements
This work was carried out with the financial support from
the Ministry of Economy and Competitiveness (MINECO) of the
Spanish Government (Project CTQ2010-19188), from the Basque
Government (Project IT748-13) and from the University of the
Basque Country (UFI 11/39). Eva Epelde is grateful for the Ph.D.
grant from the Department of Education, University and Research
of the Basque Country (BFI08.122) and for the University of the
Basque Country. Technical and human support provided by SGIker
(UPV/EHU) is gratefully acknowledged.
In order to explore the possibilities for this purpose, new exper-
iments have been carried out for 15 h time on stream with HZ-30,
1
P/HZ-80 and HZ-80-ST catalysts, for which the maximum propyl-
ene yield was not obtained after 5 h time on stream. The selection of
the catalysts is based on the possibility for obtaining a high propyl-
ene yield in view of the promising results shown in Figs. 4b and 5b.
Fig. 8 shows the new results of evolution with time on stream of
ethylene conversion (Fig. 8a) and propylene yield (Fig. 8b).
HZ-30 parent catalyst is the less stable and allows obtaining
a maximum propylene yield (29%) at 6 h time on stream. HZ-80
catalyst doped with P (1P/HZ-80) is more stable, with a maximum
propylene yield of 34.1% at 8.7 h. HZSM-5 catalyst treated by mild
in situ steaming (HZ-80-ST) shows the best performance with the
highest propylene yield (37%) at the highest time on stream (11.3 h).
This result is important for its use in the fluidized bed reactor with
catalyst circulation, because as the average residence time of the
catalyst is increased, the time of catalyst usage also increases and
the regeneration costs decrease.
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