Influence of steaming on the acidity and the methanol conversion reaction of HZSM-5 zeolite

Article abstract of DOI:10.1016/j.jcat.2013.07.021

The influence of steaming at varying temperatures on the physicochemical properties of a HZSM-5 zeolite (Si/Al = 27) was investigated by X-ray diffraction, ²⁷Al MAS NMR, Ar physisorption and IR spectroscopy of adsorbed pyridine, 2,4,6-trimethylpyridine and CO. The catalytic activity of the zeolites was evaluated in propane and methanol conversion reactions. Mild steaming did not result in removal of framework Al atoms. Instead, evidence was found that some extra-framework Al atoms present in the parent zeolite were inserted into the framework at defect sites. This resulted in higher propane conversion rates. Severe steaming resulted in a strong decrease in the framework Al content and agglomeration of extra-framework Al atoms. This caused a strong decrease in the Bronsted acidity probed by pyridine and CO IR. The rate of propane conversion was consequently adversely affected. The steaming procedures did not result in the formation of noticeable mesoporosity. By IR spectroscopy of adsorbed 2,4,6-trimethylpyridine, however, indications were found for structural damage at the outer region of the zeolite crystals, resulting in increased accessibility of the Bronsted acid sites (BAS). For methanol conversion at 350 C, the concentration of BAS governs the catalytic performance. For zeolites with a high BAS density (parent and mildly steamed zeolites), the rate of deactivation is high. The total amount of methanol converted per BAS is relatively low because of the high rate of consecutive reactions. These reactions involve the conversion of the dehydration product of methanol, dimethyl ether, to products with carbon-carbon bonds and the formation of carbonaceous deposits, which deactivate the zeolite catalyst. Decreasing Bronsted acidity by severe steaming results in an increased amount of methanol converted per BAS because of the lower coke formation rate. As a result, the total amount of methanol converted per BAS increases strongly with decreasing BAS density. However, it also causes lower rates of conversion of dimethyl ether to useful products. In terms of the total amount of methanol converted per BAS to light olefins, the set of zeolites shows optimum performance at intermediate BAS density (HZSM-5 severely steamed at 450 C; concentration of BAS ～0.2 mmol g^-1).

Full text of DOI:10.1016/j.jcat.2013.07.021

Journal of Catalysis 307 (2013) 194–203
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Influence of steaming on the acidity and the methanol conversion
reaction of HZSM-5 zeolite
⇑
Sami M.T. Almutairi, Brahim Mezari, Evgeny A. Pidko, Pieter C.M.M. Magusin, Emiel J.M. Hensen
Schuit Institute of Catalysis, Laboratory of Inorganic Materials Chemistry, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
The influence of steaming at varying temperatures on the physicochemical properties of a HZSM-5 zeolite
(Si/Al = 27) was investigated by X-ray diffraction, 27Al MAS NMR, Ar physisorption and IR spectroscopy of
adsorbed pyridine, 2,4,6-trimethylpyridine and CO. The catalytic activity of the zeolites was evaluated in
propane and methanol conversion reactions. Mild steaming did not result in removal of framework Al
atoms. Instead, evidence was found that some extra-framework Al atoms present in the parent zeolite
were inserted into the framework at defect sites. This resulted in higher propane conversion rates. Severe
steaming resulted in a strong decrease in the framework Al content and agglomeration of extra-frame-
work Al atoms. This caused a strong decrease in the Brønsted acidity probed by pyridine and CO IR.
The rate of propane conversion was consequently adversely affected. The steaming procedures did not
result in the formation of noticeable mesoporosity. By IR spectroscopy of adsorbed 2,4,6-trimethylpyri-
dine, however, indications were found for structural damage at the outer region of the zeolite crystals,
resulting in increased accessibility of the Brønsted acid sites (BAS). For methanol conversion at 350 °C,
the concentration of BAS governs the catalytic performance. For zeolites with a high BAS density (parent
and mildly steamed zeolites), the rate of deactivation is high. The total amount of methanol converted per
BAS is relatively low because of the high rate of consecutive reactions. These reactions involve the con-
version of the dehydration product of methanol, dimethyl ether, to products with carbon–carbon bonds
and the formation of carbonaceous deposits, which deactivate the zeolite catalyst. Decreasing Brønsted
acidity by severe steaming results in an increased amount of methanol converted per BAS because of
the lower coke formation rate. As a result, the total amount of methanol converted per BAS increases
strongly with decreasing BAS density. However, it also causes lower rates of conversion of dimethyl ether
to useful products. In terms of the total amount of methanol converted per BAS to light olefins, the set of
zeolites shows optimum performance at intermediate BAS density (HZSM-5 severely steamed at 450 °C;
concentration of BAS ꢀ0.2 mmol gꢁ1).
Received 21 May 2013
Revised 16 July 2013
Accepted 21 July 2013
Keywords:
Zeolite
HZSM-5
Acidity
Steaming
Methanol
Ó 2013 Elsevier Inc. All rights reserved.
1. Introduction
chelating agents like EDTA [9] and (NH4)2SiF6 [10], or leaching with
HCl [11–13]. Typically, the removal of Al from the framework
Zeolites are crystalline microporous aluminosilicates, which are
extensively used in industry to catalyze a wide variety of reactions.
The acid catalytic activity of zeolite resides in tetrahedral frame-
work Al atoms. The charge-compensating protons of the oxygen
anions bridging between Al and Si give rise to strong Brønsted
acidity [1]. Hydrolytic removal of Al atoms from their framework
sites (dealumination) is an important method to modify the cata-
lytic properties of zeolites. Dealumination by steam calcination is
a key technology in the manufacture of strongly acidic and acces-
sible faujasite-based cracking zeolites [2–5]. Besides conventional
(steam) calcination [6,7], dealumination can also be done by
post-synthesis modification such as treatment with SiCl4 [8],
results in lower density of Brønsted acid sites (BAS). The extra-
framework Al atoms may have a positive influence on the intrinsic
acidity of the zeolite [14–16].
Typically, high-silica zeolites such as HZSM-5 are more stable
than low-silica ones such as faujasites, thus requiring more severe
conditions to dealuminate them [17,18]. Enhanced cracking activ-
ity of mildly steamed ZSM-5 has been attributed to the presence of
extra-framework Al (EFAl) [14,19,20]. The nature of such EFAl
atoms and the way they affect acid activity is not completely
understood. Several types of octahedral Al atoms have been iden-
tified by 27Al MAS NMR spectroscopy [21]. In addition, tetrahedral
Al atoms may be present at ion-exchange positions and they may
be invisible in NMR spectra due to strong quadrupolar broadening
[22]. Neutral species such as monomeric AlOOH, Al(OH)3 [23],
aluminum-oxo and hydroxo clusters [24–27], and bulk aluminum
⇑
Corresponding author.
E-mail address: e.j.m.hensen@tue.nl (E.J.M. Hensen).
0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.07.021

S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
195
oxide aggregates have also been reported for dealuminated zeolites
[28,29]. The extraction of framework Al may also affect the pore
structure and porosity by generation of voids larger than the origi-
nal micropores. In addition, pores may become blocked by aggre-
gated forms of Al. Usually, the number of Brønsted acid sites in
HZSM-5 decreases with increasing dealumination time [30].
Lercher and co-workers showed that at relatively low steaming
temperature (450 °C), isolated Al atoms are more prone to extrac-
tion from the lattice than Al atoms in close proximity to other Al-
occupied tetrahedra [31]. In some cases, enhancement of protolytic
n-alkane cracking has been reported upon mild steaming of HZSM-
5 [14,32], which has been ascribed to synergy between Brønsted
and Lewis acid sites [14]. Besides changes in the number and
strength of the BAS, also their location is of crucial importance to
the final catalytic performance. Dealumination can also result in
surface roughening of the zeolites crystals and mesoporosity
[33]. Mild steaming results in mesoporosity at the perimeter of
the zeolite crystals, whereas steaming at 700 °C results in uni-
formly distributed mesopores. Severe dealumination of HZSM-5
suppresses coke formation and increases the performance during
methanol conversion reactions [34]. The formation of carbona-
ceous residue in the course of the catalytic reaction deactivates
the internal BAS. For methanol conversion, it is advantageous to
have a low density of BAS because it lowers the rate of coke forma-
tion [35,36].
(Bruker) was used to calculate the crystallinities of the zeolite sam-
ples from the XRD patterns.
2.2.2. Ar physisorption measurements
Ar sorption measurements were performed at ꢁ186 °C on a
Micromeritics ASAP-2020 apparatus in static measurement mode.
In a typical experiment, a zeolite sample (ꢀ100 mg) was outgassed
at 350 °C for 8 h prior to the sorption measurement. The Brunauer–
Emmett–Teller (BET) equation was used to calculate the specific
surface area from the adsorption data obtained in the p/p0 range
of 0.05–0.25. The volume of mesopores (Vmeso) and the area of mes-
opores (Smeso) were calculated using the Barrett–Joyner–Halenda
(BJH) method on the adsorption branch of the isotherm. The micro-
pore area (Smicr) and micropore volume (Vmicr) were calculated,
employing the t-plot method using the thickness range between
3.4 and 5.0 Å.
2.2.3. Thermogravimetric analysis (TGA)
TGA measurements were performed on Mettler TGA/DSC-1 con-
nected to gas analysis system (ThermoStar™) using the following
conditions: 70 ll alumina crucibles, dry air as a purge gas, and
nitrogen as a protective gas with flows of 40 ml minꢁ1 each. In a
typical experiment, the temperature was increased from 40 to
850 °C at a rate of 5 °C minꢁ1
.
Herein, we investigate the influence of steaming on the acidic
properties of HZSM-5 zeolite and the methanol conversion reac-
tion. For this purpose, the steam flow and steam temperature were
varied. The parent and modified ZSM-5 zeolites were characterized
by XRD, Ar physisorption, 27Al MAS NMR spectroscopy and FTIR
spectroscopy of pyridine, 2,4,6-trimethylpyridine, and CO. The cat-
alytic activities of these zeolites are compared in propane and
methanol conversion.
2.2.4. 27Al MAS NMR spectroscopy
Magic angle spinning (MAS) 27Al single pulse NMR spectra were
recorded on a Bruker DMX500 NMR spectrometer operating at a
magnetic field of 11.7 T. The 27Al NMR frequency was 130 MHz.
The 27Al chemical shift is referred to a saturated Al (NO3)3 solution.
In a typical experiment, about 10 mg of well-hydrated sample was
accurately weighed and packed in a 2.5-mm zirconia rotor. The
sample rotation speed was 20 kHz. Single-pulse excitation was
used with a 20° pulse of 1
allowed quantitative measurements as judged from test spectra
with longer pulse duration (2 s) or relaxation delay (2 s). Prior
ls and a relaxation delay of 1 s, which
l
2. Experimental procedures
to NMR measurements, the samples were hydrated in a desiccator.
NMR spectra were also recorded after treating the mildly dried
zeolite in a flow of ammonia (10% NH3 in He) at 120 °C. These sam-
ples are denoted by the use of the suffix –NH3.
2.1. Synthesis of materials
The parent NH4 form of ZSM-5 (SM-55) zeolite (SAR = 55) was
obtained from Süd-Chemie. The proton form of the zeolite was ob-
tained by calcination in a mixture of 20 vol.% oxygen in nitrogen at
a flow rate of 100 Nml/min, while heating to 550 °C at a rate of
5 °C/min followed by an isothermal period of 6 h. This parent zeo-
lite was further treated by mild and severe steaming with 100%
water vapor. A first set was prepared by heating zeolite (in 1 g
batches) to the steaming temperature under He flow, followed by
steaming at a total flow rate of 8.2 ꢂ 10ꢁ5 mol minꢁ1 at 400, 450,
500, 550, and 600 °C for 6 h. These zeolites are denoted by
HZSM-5(ms, x) with ms referring to mild steaming and x to the final
steaming temperature in °C. For severe steaming, a higher steam
flow rate of 3.3 ꢂ 10ꢁ4 mol minꢁ1 was employed under otherwise
similar conditions. These zeolites are denoted by HZSM-5(ss,x).
The steaming treatments were performed in a fixed-bed calcina-
tion oven (shallow bed).
2.2.5. Infrared spectroscopy of adsorbed pyridine, 2,4,6-
trimethylpyridine, and CO
Brønsted and Lewis acidity of the zeolites was investigated by
FTIR spectroscopy of adsorbed pyridine and 2,4,6-collidine (2,4,6-
trimethylpyridine). FTIR spectra were recorded in a Bruker Vertex
V70v FTIR spectrometer in transmission mode. In a typical experi-
ment, a zeolite sample was pressed into a self-supporting wafer
with a density of 5–6 mg cmꢁ2. The wafer was placed in a
controlled-environment transmission cell equipped with CaF2
windows. Prior to recording the spectra, the zeolite was dehy-
drated in an oxygen flow under heating from ambient temperature
to 550 °C at a rate of 5 °C minꢁ1. The sample was then evacuated at
550 °C for 0.5 h and cooled to 150 °C temperature in vacuum. Dur-
ing this pretreatment, the cell body and the evacuated gas-supply
section of the setup were kept at 80 °C to remove remaining phys-
isorbed water. The final pressure of the cell was lower than
2 ꢂ 10ꢁ6 mbar. The zeolite wafer was then exposed to pyridine at
150 °C. After a hold time of 20 min, the sample was evacuated
for 1 h to remove physisorbed adsorbate and a first spectrum
was recorded. Subsequently, the sample was heated to 350 °C, kept
for 1 h at this temperature, and cooled to 150 °C. Then, a second
spectrum was recorded. Finally, the sample was heated to 500 °C,
kept for 1 h at this temperature, and cooled to 150 °C. Then, a third
spectrum was recorded. Total Brønsted and Lewis acidity and the
concentrations of weak, medium, and strong forms of these sites
2.2. Catalyst characterization
2.2.1. X-ray diffraction (XRD)
The integrity of the zeolite structure was characterized by X-ray
diffraction (XRD). XRD patterns were recorded on a Bruker D4 En-
deavor Diffractometer using Cu Ka radiation with a wavelength of
1.54056 Å. 2h angles from 5° to 60° were measured with a step size
of 0.077° and a time per step of 1 s. The catalysts were ground and
pressed in sample holders for measurements. The Topas software

196
S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
were derived from these data. The concentrations were calculated
on the basis of the molar extinction coefficient given by Datka et al.
3. Results
of 0.73 and 1.11 cm l
molꢁ1 for BAS and LAS, respectively [37]. For
3.1. Structural characterization
some samples, additional IR experiments with 2,4,6-collidine were
carried out in a similar fashion as done for pyridine. In this case,
the strong Brønsted acid sites are reported on the basis of the
intensity of the 1636 cmꢁ1 band after evacuation at 500 °C. The
extinction coefficient of 10.1 cm
et al. [38] was used.
For FTIR spectroscopy of adsorbed CO, the dehydrated catalyst
wafer was cooled by flowing liquid nitrogen through a capillary
spiraled around the catalyst wafer. After reaching liquid N2 tem-
perature, an initial spectrum was recorded. CO (Praxair, 99.999%)
was dosed through a sample loop connected to a 6-way valve,
allowing dosing accurate amounts of CO (0.04 mmol) to the cell
The XRD patterns (see Supporting information, Fig. S1) show
that the structure of HZSM-5 is not significantly affected by the
steaming treatment. All zeolites retain the MFI topology and their
initial high crystallinity (Table S1). The lattice parameters and the
unit cell volumes do not vary significantly for the mildly steamed
HZSM-5 samples. Steaming under severe conditions shows a more
substantial contraction of the lattice parameters and unit cell vol-
ume, especially at higher steaming temperatures. These changes
point to the removal of Al from the zeolite framework.
The textural properties of the parent and treated ZSM-5 zeolites
are given in Table 1. The BET surface areas of the steamed zeolites
are lower than those of the parent material. Typically, the micro-
pore volume and surface area decreased because of the steaming
treatment. However, there is no clear trend with the severity of
the treatment, except that severe steaming at elevated tempera-
tures leads to a slightly stronger decrease in the surface area. The
textural properties do not evidence the formation of significant
mesoporosity in the treated zeolites.
l
molꢁ1 reported by Nesterenko
in
a computer-controlled sequence. Difference spectra were
obtained by subtracting the initial spectrum of the dehydrated cat-
alyst from the spectra obtained at increasing CO coverage. Spectra
were normalized to the catalyst wafer weight.
2.2.6. Propane cracking
The acid activity of the zeolites was determined by measuring
the rate of monomolecular cracking of propane. Catalytic activity
measurements were performed in an atmospheric-pressure sin-
gle-pass quartz microflow reactor at 550 °C. The feed mixture
was delivered by thermal mass flow controllers and consisted of
5 vol.% C3H8 in He at a total flow rate of 100 ml/min. The WHSV
space velocity was kept at 11.7 hꢁ1. The product composition
was analyzed by an online three-column gas chromatograph (Com-
pact GC Interscience) equipped with a PLOT Al2O3/KCl column with
a flame ionization detector and Molsieve-5 Å and RTX-1 columns
both employing thermal conductivity detectors. The conversion
was kept below 5% to ensure differential conditions. The reaction
27Al MAS NMR spectra for the parent (HZSM-5) and steamed
zeolites are shown in Fig. 1. The spectra clearly show that the main
effect of the steaming treatment is partial extraction of framework
Al atoms to extra-framework positions. The spectrum of the parent
HZSM-5 zeolite contains a strong signal around 55 ppm corre-
sponding to tetrahedral framework Al atoms and a weak one
around 0 ppm corresponding to extra-framework Al atoms in octa-
hedral coordination. The intensity of the peak around 55 ppm de-
creased slightly with steaming temperature for the mildly
steamed zeolites. The loss of framework Al was not quantitatively
compensated by an increase in the signal of extra-framework spe-
cies (cf. Alvisible in Table 2), implying that NMR-invisible Al nuclei
are formed. The dealumination degree did not appreciably change
anymore when the steaming treatment was done at temperatures
above 500 °C. The results are different when steaming is carried
out under severe conditions. The intensity of the signal due to tet-
rahedral Al decreased progressively with increasing steaming tem-
perature. A broad peak at 30 ppm appeared in the NMR spectra,
which is assigned to framework Al atoms in a distorted tetrahedral
coordination or penta-coordinated Al atoms.
F
rates (r) were calculated by use of r ¼ X: m , where X is conversion
cat
of propane, F is flow rate in mol sꢁ1, and mcat is the weight of the
catalyst in g.
2.2.7. Methanol conversion
The catalytic activity in the methanol to olefins reaction was
determined in a quartz tubular fixed-bed reactor. The catalyst
was placed as pressed, crushed, and sieved particles in the 125–
Treatment in NH3 led to complete disappearance of AlVI in the
spectra of HZSM-5(ms, 500), HZSM-5(ms, 550) and HZSM-5(ms,
600) (Fig. 2), evidencing that the octahedral Al atoms in these sam-
ples are still closely associated with the zeolite framework [39,40].
In contrast, the spectra of ammonia-treated severely steamed zeo-
lites showed much less changes in the Al coordination, indicating
that in this case, the Al atoms were dislodged from the framework
and are part of a separate aluminum-rich phase. These qualitative
trends are confirmed by the data reported in Table 2 obtained by
250
lm range between two quartz wool plugs. Prior to reaction,
the catalyst was activated at 550 °C in artificial air (30 ml minꢁ1
)
for 2 h. The methanol conversion was performed at 350 °C. Metha-
nol (Merck, 99%) was introduced to the reactor by flowing He
through a saturator containing the reactant. The WHSV was kept
at 6:65 gMeOH gꢁca1t ꢁ1, and the effluent was analyzed by online gas
h
chromatography (Compact GC, Interscience) equipped with TCD
and FID detectors with RT-Q-Bond and Al2O3/KCl columns,
respectively.
Table 1
Textural properties of ZSM-5 and steamed ZSM-5 zeolites as determined by Ar physisorption.
Sample
SBET (m2 gꢁ1
)
Smeso (m2 gꢁ1
)
BJHmeso (m2 gꢁ1
)
Vmeso (cm3 gꢁ1
)
Smicro (m2 gꢁ1
)
Vmicro (cm3 gꢁ1
)
HZSM-5
326
267
270
n.d.
270
291
291
291
293
257
238
37
31
34
n.d.
26
31
38
39
39
32
27
19
13
15
n.d.
12
12
16
17
17
14
13
0.033
0.026
0.027
n.d.
0.023
0.025
0.031
0.034
0.036
0.028
0.026
288
236
236
n.d.
244
260
253
253
254
225
210
0.14
0.11
0.11
n.d.
0.12
0.12
0.12
0.12
0.12
0.11
0.1
HZSM-5(ms, 400)
HZSM-5(ms, 450)
HZSM-5(ms, 500)
HZSM-5(ms, 550)
HZSM-5(ms, 600)
HZSM-5(ss, 400)
HZSM-5(ss, 450)
HZSM-5(ss, 500)
HZSM-5(ss, 550)
HZSM-5(ss, 600)

S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
197
55 ppm
55 ppm
50 ppm
0 ppm
0 ppm
(ss, 600)
(ss, 550)
(ss, 500)
(ss, 450)
(ss, 400)
(ms, 600)
(ms, 550)
(ms, 500)
(ms, 450)
(ms, 400)
HZSM-5
HZSM-5
100
80
60
40
20
0
-20
100
80
60
40
20
0
-20
27
Al Chemical shift (ppm)
27
Al Chemical shift (ppm)
Fig. 1. 27Al MAS NMR spectra of HZSM-5 treated under mild (left) and severe (right) steaming conditions.
concentration of BAS is observed after steaming at 550 °C. The
increase in the BAS concentration goes together with a small
decrease in the concentration of LAS. This may indicate that the
steaming procedure causes reinsertion of extra-framework Al spe-
cies into the framework or relocation/redispersion of such species
to positions where addition of a nitrogen-containing base leads to
reinsertion. Severe steaming results in considerable decrease in the
total acidity. The higher the steaming temperature, the stronger
the decrease in acidity. This is consistent with the trend in frame-
work Al content determined by 27Al NMR.
The presence of BAS at the external zeolite surface of the parent
zeolite and of a subset of the steamed zeolites was probed by IR
spectroscopy of adsorbed 2,4,6-collidine. In this case, only a single
IR band around 1635 cmꢁ1 due to protonated 2,4,6-collidine was
observed. The concentrations of external BAS are given in Table 3.
The data show an increasing amount of BAS accessible to 2,4,6-col-
lidine with the severity of the steaming treatment. This trend is at
odds with the one determined by pyridine IR. A tentative explana-
tion is that framework damage close to the pore openings because
of the steam treatment causes a slightly better accessibility of BAS
close to the external surface.
Table 2
Al distribution and relative amount of Al species in the HZSM-5 and steamed ZSM-5
as determined by 27Al NMR spectroscopy.
Sample
AlIV (%)
Aldistorted (%)
AlVI (%)
Alvisible (%)
HZSM-5
HZSM-5-NH3
93
100
92
91
90
100
93
99
90
100
67
56
47
68
40
54
40
37
0
0
0
0
0
0
0
0
0
7
0
8
9
10
0
100
100
90
95
86
89
88
90
85
88
65
60
59
77
57
69
56
61
HZSM-5(ms, 400)
HZSM-5(ms, 450)
HZSM-5(ms, 500)
HZSM-5(ms, 500)-NH3
HZSM-5(ms, 550)
HZSM-5(ms, 550)-NH3
HZSM-5(ms, 600)
HZSM-5(ms, 600)-NH3
HZSM-5(ss, 400)
HZSM-5(ss, 450)
HZSM-5(ss, 500)
HZSM-5(ss, 500)-NH3
HZSM-5(ss, 550)
HZSM-5(ss, 550)-NH3
HZSM-5(ss, 600)
HZSM-5(ss, 600)-NH3
7
1
10
0
16
20
24
9
30
30
30
26
0
17
24
28
23
30
16
30
35
The presence and strength of BAS and LAS in zeolite were also
determined by IR spectroscopy of adsorbed CO (Fig. 3). At low tem-
perature, the weak base CO can polarize the bridging hydroxyl
groups. The strength of the BAS can be determined by measuring
deconvolution of the single pulse NMR spectra. The parent and the
mildly steamed HZSM-5 zeolites all contain about 10% octahedral
Al. All these species show flexible Al coordination. A small increase
in the amount of NMR-invisible Al is noted with increasing steam-
ing temperature. The severely steamed zeolites, however, contain a
more significant fraction of octahedral and distorted tetrahedral/
five-coordinated Al atoms. Their fraction strongly increases with
the steaming temperature. Upon ammonia treatment, only some
non-tetrahedral Al atoms revert their coordination to tetrahedral,
indicating segregation of Al from the framework to a separate
phase. This segregation of Al atoms becomes more significant with
increasing steaming temperature.
the stretching vibrations of the perturbed carbonyl,
the perturbed hydroxyl groups, (OH). The interaction between
the proton and CO results in a red shift of (OH) and a blue shift
of (CO). The stronger these shifts, the stronger the Brønsted acid-
m(CO), and of
m
m
m
ity of the hydroxyl group. The difference IR spectra of (steamed)
HZSM-5 after CO adsorption are shown in Fig 3. The most intense
feature in the CO stretching region of HZSM-5 is observed at
2174 cmꢁ1, which is ascribed to CO adsorbed on strong BAS. The
formation of CO condensed in the zeolite micropores results in a
broad band with a maximum at 2138 cmꢁ1. In the hydroxyl-
stretching region, two negative bands are observed at 3668 and
3616 cmꢁ1, which are respectively assigned to extra-framework
Al hydroxide (Al–OH) and bridging hydroxyl (Si–OH–Al) groups.
At these CO coverages, the non-acidic silanol groups are hardly
perturbed. The OH stretching band is shifted to 3303 cmꢁ1, which
corresponds to strong BAS. The spectra of the mildly steamed zeo-
lites are qualitatively similar to those of the parent zeolite. The OH
shift upon CO perturbation is very similar, indicating that the acid
strength has not been changed due to mild steaming. A difference
noted in the CO IR spectra of HZSM-5, HZSM-5(ms, 550), and
HZSM-5(ms, 600) is the higher intensity of the bands related to
the bridging hydroxyl groups for the latter two zeolites. This differ-
ence is consistent with the increase in BAS probed by pyridine IR.
The spectra for the severely steamed zeolites (Figs. 3d and e) exhi-
bit differences that are more distinct. Firstly, the spectra contain an
3.2. Acidity characterization
The concentration and strength of the Brønsted acid sites and
Lewis acid sites (LAS) was determined by IR spectroscopy of
adsorbed pyridine. The acidic properties of the parent and steamed
HZSM-5 zeolites are summarized in Table 3. The total BAS concen-
tration for the parent zeolite is about 530
at 300 °C, most of these sites are still covered with pyridine
(ꢀ 500 mol/g). The amount of LAS is in the range 70–110 mol/
g. Accordingly, the total acidity is close to 550–600 mol/g in rea-
lmol/g. After evacuation
l
l
l
sonable agreement with the total Al content of the parent HZSM-5
zeolite. Care should be taken with the values determined by pyri-
dine IR, as there is a large spread in reported values for the extinc-
tion coefficients for the pyridine ring vibrations representing BA
and LA sites [41]. For mildly steamed zeolites, a maximum in the

198
S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
(ms,500)
(ms, 550)
(ms, 600)
(ss, 500)
(ss, 550)
(ss, 600)
100
80
60
40
20
0
-20
100
80
60
40
20
0
-20
100
80
60
40
20
0
-20
27
27
27
Al chemical shift (ppm)
Al chemical shift (ppm)
Al chemical shift (ppm)
Fig. 2. 27Al MAS NMR spectra before (full lines) and after (dashed lines) ammonia exposure of a selection of steamed ZSM-5 zeolites.
Monomolecular cracking of propane at 550 °C was used as a test
reaction to evaluate the acid catalytic activity of the zeolites. The
Table 3
Concentration of Brønsted and Lewis acid sites (IR spectroscopy of adsorbed pyridine)
and external Brønsted acid sites (IR spectroscopy of adsorbed 2,4,6-collidine).
conversion was kept below 5% in all cases. Accordingly, hydride
transfer and secondary reactions are not expected to contribute
to the catalytic performance. This is confirmed by the finding that
the molar ratio of product ethylene and methane is close to unity
for all zeolites. After the reaction, the catalyst was white, indicating
that no significant amounts of coke deposits were formed. Table 4
collects the activity results, the overall reaction rate, the rates of
propane cracking, and propane dehydrogenation. For propane
cracking, the products are methane and ethylene. Propylene is
the hydrocarbon product used to calculate the rate of propane
dehydrogenation. The activities of the steamed HZSM-5 zeolites
were different from the parent zeolite. Firstly, it is noted that
among the mildly steamed zeolites, the highest activity is observed
for HZSM-5(ms, 550). For this zeolite, both the overall conversion
rate and the cracking and dehydrogenation activities are signifi-
cantly higher than that of the parent zeolite. The activities of the
zeolites steamed at lower temperatures are comparable to the ref-
erence, except for HZSM-5(ms, 400) with a much lower activity.
The reason for this remains unclear especially in view of the results
of pyridine IR, indicating that Brønsted acidity of this sample was
unaltered. All of the severely steamed zeolites are less active in
propane conversion than the parent zeolite. With increasing
steaming temperature, the propane conversion and the rates of
cracking and dehydrogenation strongly decrease in line with the
spectroscopic results showing dislodging of Al from the framework
and loss of strong Brønsted acid sites.
Sample
BAS (
l
mol/g)a LAS (
l
mol/g)a External BAS ( mol/g)a
l
HZSM-5
530
111
116
117
92
90
88
111
84
51
75
62
18
21
HZSM-5(ms, 400) 533
HZSM-5(ms, 450) 540
HZSM-5(ms, 500) 652
HZSM-5(ms, 550) 814
HZSM-5(ms, 600) 668
HZSM-5(ss, 400)
HZSM-5(ss, 450)
HZSM-5(ss, 500)
HZSM-5(ss, 550)
HZSM-5(ss, 600)
n.d.b
n.d.
23
n.d.
27
n.d.
n.d.
29
393
237
130
128
126
n.d.
a
Determined after evacuation at 150 °C.
Not determined.
b
additional band at 2168 cmꢁ1, which becomes more dominant at
higher steaming temperature. This band is due to extra-framework
AlOH–CO interactions [42]. In the hydroxyl region, a perturbed OH
band in the region 3450–3500 cmꢁ1 appears due to weaker BAS,
likely related to extra-framework AlOH groups. Note that in the
spectra of the parent and mildly steamed zeolites such bands are
also present in the form of very weak shoulders. Secondly, a small
number of weak Lewis acid sites are noticeable as follows from the
appearance of a shoulder around 2190 cmꢁ1. Importantly, the red
shift of
m(OH) upon CO perturbation is similar for all zeolites,
implying that the intrinsic acidity of the BAS in the various zeolites
is similar. The COads IR spectra do not evidence the presence of
large amounts of Lewis acid sites. This may be taken as an indica-
tion that the dispersion of the extra-framework Al phase is rela-
tively low.
3.3. Catalytic activity measurements
Fig. 4 shows the activities of the parent and steamed zeolites in
the methanol conversion reaction as a function of the time on

S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
199
3303
2174
(a)
(a)
0.4
0.2
0.4
0.2
2138
0.0
-0.2
3616
0.0
2250
3800
3600
3600
3600
3600
3600
3400
3200
3200
3200
3200
3200
3000
2200
2200
2200
2200
2200
2150
2100
2100
2100
2100
2100
2050
0.6
0.4
0.2
0.0
-0.2
0.6
0.4
0.2
0.0
3305
(b)
(b)
2173
3616
3800
3400
3400
3400
3400
3000
2250
2150
2050
3304
(c)
(c)
0.4
0.2
0.4
0.2
0.0
0.0
-0.2
3616
3800
3000
2250
2150
2165
2050
2173
(d)
(d)
0.4
0.2
3304
0.4
0.2
0.0
3466
0.0
-0.2
3616
3800
3000
2250
2150
2050
0.10
(e)
(e)
0.4
0.2
0.0
3466
3504
0.05
3305
2165
0.00
3619
-0.05
-0.10
3800
3000
2250
2150
2050
Fig. 3. Infrared spectra of the hydroxyl (left) and CO (right) stretching regions upon progressive adsorption of CO at 80 K for (a) HZSM-5, (b) HZSM-5(ms, 550), (c) HZSM-
5(ms, 600), (d) HZSM-5(ss, 400), and (e) HZSM-5(ss, 600).
stream. The conversions and carbon selectivities are summarized
in Table 5. For determination of the methanol conversion and
selectivities, dimethyl ether was considered to be reactant. The
mildly steamed catalysts showed almost similar performance in
the methanol conversion reaction as the parent zeolite. After 1 h,
the methanol conversion for all these samples is close to 100%.

200
S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
Table 4
Catalytic conversion of propane over hydrogen forms of ZSM-5 zeolites (WHSV = 11.7 hꢁ1; T = 550 °C).
Sample
roverall ꢂ 10ꢁ5 (mol gꢁ1 sꢁ1
)
rCRC ꢂ 10ꢁ5 (mol gꢁ1 sꢁ1
)
rdeh ꢂ 10ꢁ5 (mol gꢁ1 sꢁ1
)
r
deh/rCRC ratio
HZSM-5
23.6
14.1
20.2
25.3
32.3
27.2
21.4
16.6
14.9
8.72
6.94
20.4
12.4
17.7
21.8
28.0
23.4
17.5
14.5
13.0
7.49
5.88
3.19
1.76
2.52
3.41
4.17
3.80
3.88
2.05
1.90
1.24
1.07
0.16
0.14
0.14
0.16
0.15
0.16
0.22
0.14
0.15
0.17
0.18
HZSM-5(ms, 400)
HZSM-5(ms, 450)
HZSM-5(ms, 500)
HZSM-5(ms, 550)
HZSM-5(ms, 600)
HZSM-5(ss, 400)
HZSM-5(ss, 450)
HZSM-5(ss, 500)
HZSM-5(ss, 550)
HZSM-5(ss, 600)
100
100
HZSM-5
(ss, 400)
(ss, 450)
(ss, 500)
(ss, 550)
(ss, 600)
HZSM-5
(ms, 400)
80
60
40
20
0
80
(ms, 450)
(ms, 500)
(ms, 550)
60
(ms, 600)
40
20
0
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
Time on stream (h)
Time on stream (h)
Fig. 4. Methanol conversion over the parent HZSM-5 zeolite and HZSM-5 after mild (left) and severe (right) steaming (WHSV = 6:65 gMeOH gꢁ1 hꢁ1; T = 350 °C). The open
cat
symbols indicate the regime where the rate of deactivation is smaller than 0.5%/h.
The propylene selectivity is slightly higher than the ethylene selec-
tivity. Other products are higher hydrocarbons including some aro-
matics. The combined ethylene and propylene selectivity is around
60–65% for the mildly steamed samples and increases up to nearly
90% for the less acidic severely steamed ones. In addition, a sub-
stantial amount of propane is produced. After an initial period of
complete methanol conversion (except for several weakly acidic
zeolites), the methanol conversion decreases for all zeolites. Ini-
tially, the reactor effluent did not contain methanol and dimethyl
ether, implying their complete conversion to products with car-
bon–carbon bonds. Thereafter, methanol was first observed in
the reaction mixture and only at a somewhat later stage dimethy-
lether. The concentration of methanol in the reactor effluent in-
creased much faster with time on stream than the concentration
of dimethylether. This suggests that methanol is directly converted
into products with carbon–carbon bonds. Typically, only when the
methanol conversion had significantly decreased, equilibrium was
established between methanol and dimethylether. For the least
acidic zeolite HZSM-5(ss, 600), equilibrium between methanol
and dimethylether was established already at the start of the cat-
alytic experiment. For all catalysts, it is observed that a regime of
strong catalyst deactivation is followed by residual activity in
methanol conversion (indicated with open markers in Fig. 4). The
low activity regime is characterized by a low rate of deactivation,
which increased selectivity to lighter products at the expense of
higher hydrocarbons. It is likely that the active sites for methanol
conversion in this case are located at the outer region or the exter-
nal surface of the zeolite crystals because carbonaceous deposits
have blocked their micropore space.
The catalytic performance is expressed as a turnover number
(TON) defined as the total amount of methanol converted into
products with carbon–carbon bonds (products different from
Table 5
hꢁ1; T = 350 °C; dimethyl ether not included in selectivities).
ꢁ1
Methanol conversion and product selectivities after 1 h time on stream (WHSV = 6:65 gMeOH
g
cat
Sample
XMeOH (%)
TONa
Coke (g/g)b
Selectivity(%)
CH4
¼
¼
C2
C2
C3
C3
C4
C5+
HZSM-5
100
98
98
99
100
69
100
95
87
49
12
1.6 ꢂ 103
1.7 ꢂ 103
1.5 ꢂ 103
1.3 ꢂ 103
1.2 ꢂ 103
0.9 ꢂ 103
6.9 ꢂ 103
>2.0 ꢂ 104
7.7 ꢂ 103
1.9 ꢂ 103
7.6 ꢂ 102
0.11
n.d.
n.d.
n.d.
n.d.
n.d.
0.08
0.06
n.d.
0.04
0.01
2.5
0.8
1.6
1.6
0.0
2.5
1.7
0.9
2.6
0.9
2.8
26
22
25
25
26
32
33
31
29
30
41
0.4
0.2
0.3
0.3
0.4
0.7
0.3
0.2
0.4
0.2
0.7
38
42
40
37
39
30
39
48
48
52
47
9
8
4
8
3
3
3
3
0.0
0.0
2
20
19
20
22
22
18
18
17
13
11
8
HZSM-5(ms, 400)
HZSM-5(ms, 450)
HZSM-5(ms, 500)
HZSM-5(ms, 550)
HZSM-5(ms, 600)
HZSM-5(ss, 400)
HZSM-5(ss, 450)
HZSM-5(ss, 500)
HZSM-5(ss, 550)
HZSM-5(ss, 600)
10
11
10
14
7
3
5
0.9
0.0
5
0.0
a
Turnover number: number of moles of methanol converted into products different from dimethyl ether per BAS; the concentration of BAS is the value from pyridine IR
after evacuation at 300 °C;
b
Determined after 34 h of reaction by TGA analysis.

S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
201
dimethyl ether) per BAS. TON is determined until stable methanol
conversion was obtained (detailed procedure outlined in Fig. S2).
The values for HZSM-5 and HZSM-5(ms, 400) are almost similar
(Table 5). The TON values decrease with increasing temperature
of mild steaming and, accordingly, with increasing concentration
of BAS. HZSM-5(ms, 600) deactivates almost instantaneously. The
changes in catalytic performance are substantially different for
the severely steamed zeolites. The methanol conversion after 1 h
for HZSM-5(ss, 400) was 100%. The conversion after 1 h decreased
significantly for the other severely steamed zeolites. The main
selectivity differences with the mildly steamed zeolites is the high-
er combined ethylene and propylene selectivity, which increases
from 72 to 88% with increasing steaming severity and lower pro-
pane selectivity. Importantly, the TON values are much higher for
all severely steamed zeolites compared to the mildly steamed ones.
For HZSM-5(ss, 450), it is more than an order of magnitude higher
than for HZSM-5. Note that this zeolite exhibit a relatively high
conversion after a reaction time of 34 h, so that its TON value rep-
resents a lower bound.
Several spent severely steamed zeolites were further analyzed
for their coke content by TGA analysis. The coke content was typ-
ically lower than for the spent parent zeolite. The coke content de-
creased with increasing steaming temperature, which can be
associated with the lower amount of methanol converted. The least
active HZSM-5(ss, 550) and HZSM-5(ss, 600) produce very small
amounts of coke. From the observation that the maximum weight
loss during air calcination occurs at 560 °C, it can be concluded that
the type of coke formed in all zeolites is similar and corresponds to
polyaromatic species.
coordination, resulting in high quadrupolar coupling constants,
which cause peak broadening outside detection in the 1D spec-
trum. These extra-framework Al atoms may be located at ion-ex-
change sites of the zeolite [31].
IR spectroscopy of adsorbed pyridine shows that the parent
HZSM-5 zeolite predominantly contains BAS next to a small
amount of LAS. Steaming at low temperatures (T < 500 °C) did
not affect the acid site density. However, when steaming was
carried out in the temperature range 500–600 °C, the BAS content
increased. Concomitantly, the LAS concentration decreases slightly.
These findings point to re-alumination of the framework. It has al-
ready been established earlier that Al atoms can insert in the
framework of an already synthesized zeolite [44]. At relatively
low temperatures (T ꢃ 350 °C), this is the result of reactions of
Al3+ with internal hydroxyl nests, whereas at higher temperatures
(T > 500 °C), direct substitution of Si4+ by Al3+ may also take place
[45]. Therefore, it is assumed that extra-framework Al atoms are
substituting Si framework atoms during mild steaming. Small
amounts of water may promote the structural reorganization of
the framework. In spite of being lower than for the parent zeolite,
the cell volume of HZSM-5(ms, 550) is the highest among the
mildly steamed zeolites, consistent with the proposal of Al inser-
tion. A further indication is the higher intensity of the bands
related to strong BAS in the IR spectra of adsorbed CO for HZSM-
5(ms, 550) as compared to HZSM-5.
When HZSM-5 was steamed severely (high steam flow), a sub-
stantial portion of the framework aluminum was removed. This is
evident from the higher fraction of AlVI and distorted Al (distorted
tetrahedral or penta-coordinated) atoms observed around 0 and
30 ppm, respectively, in the 27Al NMR spectra. It is also found that
a larger fraction of Al atoms becomes NMR-invisible. In contrast to
the mildly steamed zeolites, ammonia treatment did not lead to
significant changes in the Al speciation (cf. Fig. 2 and Table 2). From
this, it follows that a larger part of the framework Al has segregated
into an aluminum-rich phase as compared to the mildly steamed
zeolites. Clearly, the tetrahedral peak in the 1D 27Al NMR spectra
of the severely steamed samples consists of two components,
one around 56 ppm and another one around 53 ppm. The spectra
in Fig. 1 show that the former band decreases stronger at interme-
diate steaming temperatures than the latter. This observation is in
agreement with the results of Ong et al. [31]. The two resonances
are due to isolated and grouped Si(OH)Al groups, respectively.
Ong et al. found that steaming at 450 °C removed only the isolated
Al atoms. The present data show that both types of Al atoms can be
removed at higher steaming temperatures. Consistent with the
trend of increasing framework Al removal, the pyridine IR results
show a substantial decrease in Brønsted acidity with increasing se-
vere-steaming temperature. Removal of framework Al already
starts during steaming at 400 °C.
4. Discussion
The acidity of HZSM-5 zeolite can be varied by treatment in
steam at elevated temperatures. The severity of the steam treat-
ment was adjusted by varying the flow of water vapor and the tem-
perature. The XRD and physisorption data indicate that the
structure and porosity of HZSM-5 was not significantly affected
by the hydrothermal treatment. The crystallinities of all zeolite
samples remain higher than 90%. Severe steaming (high steam
flow) resulted in slightly larger changes (lower crystallinity and
cell volume) with a clear trend as a function of the treatment tem-
perature than mild steaming (low steam flow). The contraction of
the cell volume is due to the removal of Al from the zeolite lattice.
This is consistent with the higher degree of dealumination for the
severely steamed zeolites as follows from 27Al NMR spectroscopy.
The observation that the crystallinity remains high despite re-
moval of Al has been explained by Ong et al. [31]. These authors
discussed reorganization of defects sites by migration of silicic acid
species. The micropore surface area and volume of steamed zeo-
lites slightly decreased compared to the parent HZSM-5 zeolite.
Despite this, textural characterization does not evidence the gener-
ation of significant mesoporosity during such steaming treatments.
27Al NMR spectroscopy shows that steaming affected the Al
speciation in the zeolites. Compared to the parent zeolite, the
steamed zeolites contain more NMR-invisible and octahedral Al
atoms. The degree of dealumination is, however, quite small for
the mildly steamed zeolites. The octahedral Al atoms are highly
dispersed and close to the framework, following from the observa-
tion that they revert to tetrahedral positions upon exposure to
ammonia. The fraction of NMR-invisible Al atoms increased
slightly with steaming temperature. It is well established that
the second-order quadrupolar interactions of Al nuclei with local
electric field gradients can broaden the 27Al MAS NMR signal of
species with lower or distorted coordination symmetries [43].
NMR-invisible Al atoms exist in a highly distorted tetrahedral
With respect to acid activity of these zeolites, it is useful to dis-
cuss first the propane conversion data (Table 4). The highest pro-
pane conversion rate was observed for the sample mildly
steamed at 550 °C. The activity of HZSM-5(ms, 550) was signifi-
cantly higher than that of the parent zeolite. The activity increase
correlates well with the higher concentration of BAS as probed
by IR spectroscopy of adsorbed pyridine. Niwa et al. have also
found that treatment in ammonia/water at relatively low temper-
atures increases the protolytic cracking rate [32]. These authors as-
cribed the acidity increase to removal of Si from the framework,
although no clear evidence was provided for this. Earlier, such
enhancement of n-alkane cracking activity by dealumination of
HZSM-5 has also been attributed to synergy between extra-frame-
work Al and BAS [14]. Such an explanation does not appear to hold
for the present set of mildly steamed ZSM-5 zeolites. For the
severely steamed zeolites, the propane conversion rate strongly
decreased with increasing steaming temperature. This trend is

202
S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
consistent with the strong decrease in the concentration of BAS
probed by pyridine and CO IR as well as of framework Al from
NMR spectroscopy. Besides, Table 4 shows that differences in
BAS content affect the protolytic cracking and dehydrogenation
of propane in almost similar ways. In all of these cases, no signifi-
cant contribution of extra-framework Al atoms on the rate of pro-
pane dehydrogenation can be derived from the catalytic activity
data. Summarizing, a low steam flow results in very small changes
to the Al distribution in the parent HZSM-5 zeolites. If at all, some
framework defects heal, resulting in a greater number of Brønsted
acid sites. A high steam flow results in severe dislodging of Al from
the zeolite framework, resulting in significantly smaller numbers
of Brønsted acid sites. At the employed conditions, the damage to
the micropore structure of the zeolites is very limited.
The activity trends during methanol conversion with the steam-
ing severity were different. All of the catalysts deactivated with
reaction time. However, deactivation is only observed after a sub-
stantial reaction time, unless the total amount of BAS is low such as
for the severely steamed ZSM-5 zeolites, resulting in partial meth-
anol conversion. Textural characterization indicates that the vari-
ous steaming procedures of the parent HZSM-5 zeolite did not
generate noticeable mesoporosity, so that the trends are mainly
the consequence of differences in the BAS content and, possibly,
their location. The increased BAS content of the zeolites steamed
under mild conditions with temperature resulted in more rapid
deactivation. Consequently, the TON decreased with increasing
BAS content for the mildly steamed zeolites (cf. Table 5). In line
with this trend, a strong decrease in the BAS density for the se-
verely steamed zeolites resulted in better utilization of the acid
sites. The optimum is observed for HZSM-5(ss, 450) with nearly
an order of magnitude higher TON than the parent zeolite. A fur-
ther decrease in the concentration of BAS by more severe steaming
resulted in lower initial methanol conversion and decreased TON.
Fig. 5 shows the relation between TON and concentration of BAS
as well as the TON based on the total amount of methanol con-
verted (TONtotal, taken into account also dimethylether). TONtotal
increases strongly with decreasing BAS density. The difference be-
tween TONtotal and TON relates to formation of dimethyl ether. The
total amount of dimethyl ether increases when the zeolite catalysts
contain less BAS.
The present data clearly show that a high concentration of BAS
causes rapid deactivation due to the high rates of consecutive reac-
tions of olefins, resulting in carbonaceous deposits (oligomers, aro-
matics). In such case, the total amount of methanol converted is
low. The higher rate of coke formation at high BAS concentration
is well known and has been suggested to relate to proximate BAS
for multipoint adsorption [48]. Taulelle and colleagues recently
showed the importance of proximate BAS for bimolecular hydride
transfer reactions [49]. Although the authors of Ref. [47] contended
that coke formation predominantly occurs on the external surface
of HZSM-5, our correlation between TON and BAS concentration
would appear to indicate the opposite, namely that coke forms
throughout the ZSM-5 micropore space. This is further supported
by recent UV–Vis and confocal fluorescence microscopy for large
ZSM-5 crystals [50]. When the BAS content decreases, the rate of
coke formation and thus catalyst deactivation becomes lower. This
interpretation is in line with the decreasing amount of coke depos-
its for less acidic zeolites (cf. Table 5). Because of the low acidity,
the reaction is largely limited to reacting methanol to dimethyle-
ther. Therefore, formation of products with carbon–carbon bonds
is maximal at an intermediate BAS concentration. For the parent
zeolite and the methanol conversion reaction conditions used in
this study, this optimum is approximately 0.2 mmol gꢁ1
.
It is worthwhile to discuss two possible alternative explana-
tions for the lower rate of deactivation for less acidic zeolites, i.e.,
(i) lower acidity of the protons remaining after severe steaming
and (ii) preferential removal of Al at the outer zones of the zeolite
crystals. The first hypothesis can be discarded, because the pro-
pane cracking activity data correlate well with the BAS content.
This is expected, as it is well known that the acid sites in HZSM-
5 exhibit similar acidity over a wide range of Si/Al ratios [1,51].
To probe whether severe steaming has preferentially removed
strong BAS from the external zeolite surface, the IR spectroscopy
data of adsorbed 2,4,6-trimethylpyridine are instructive. The re-
sults in Table 3 show that the amount of BAS accessible to this
probe increased with steaming severity. This is most likely caused
by structural damage at the external surface of the zeolite crystals.
Ar physisorption apparently does not pick up such small changes in
the porosity. Given these results, it can be concluded that differ-
ences in the activity of steamed ZSM-5 zeolites in the methanol
conversion reaction depend predominantly on the total concentra-
tion of BAS.
Currently, methanol conversion in zeolites is mainly discussed
in terms of the hydrocarbon-pool mechanism involving
methylated aromatic species [46], although for HZSM-5 alkene
methylation has also been proposed for formation of higher hydro-
carbons [47]. The hydrocarbon-pool species undergo methylation
by methanol and/or dimethylether followed by elimination of light
olefins. These olefins can be further converted via carbenium-ion
chemistry into higher hydrocarbons and ultimately also into coke.
5. Conclusion
The influence of mild (8.2 ꢂ 10ꢁ5 mol minꢁ1, 6 h) and severe
(3.3 ꢂ 10ꢁ4 mol minꢁ1
, 6 h) steaming at varying temperatures
(400–600 °C) on the physicochemical properties of a HZSM-5 zeo-
lite (Si/Al = 27) and its acid catalytic activity in monomolecular
propane cracking and methanol conversion was investigated. Mild
steaming does not result in removal of Al atoms from the frame-
work. Instead, it was found that some extra-framework Al atoms
present in the parent zeolite were inserted in the framework at de-
fect sites, resulting in an increase in the BAS and higher propane
conversion rates. Severe steaming results in a strong decrease in
the framework Al content, agglomeration of the extra-framework
Al atoms, and decreased Brønsted acidity as probed by IR spectros-
copy of adsorbed pyridine and CO as well as catalytically by de-
creased propane conversion rates. The steaming procedures did
not result in the formation of noticeable mesoporosity. IR spectros-
copy of adsorbed 2,4,6-trimethylpyridine pointed to structural
damage at the outer zones of the zeolite crystals, resulting in in-
creased accessibility of a small part of the BAS.
severely steamed
ZSM-5
75
50
25
mildly steamed
ZSM-5
0
0.0
0.2
0.4
0.6
Brønsted acidity (mmol.g-1)
For methanol conversion (T = 350 °C), the catalytic performance
is governed by the concentration of BAS. For zeolites with a high
Fig. 5. (d) TONtotal based on total methanol converted and (j) TON based on
methanol converted to products other than dimethyl ether.

S.M.T. Almutairi et al. / Journal of Catalysis 307 (2013) 194–203
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BAS density (parent and mildly steamed zeolites), the rate of deac-
tivation is high. The total amount of methanol converted per BAS is
relatively low because of the high rate of consecutive reactions.
These reactions involve the conversion the dehydration product
of methanol, dimethyl ether, to products with carbon–carbon
bonds and the formation of carbonaceous deposits, which deacti-
vate the zeolite catalyst. Decreasing the acidity by severe steaming
resulted in increased amounts of methanol converted per BAS be-
cause of a lower rate of coke formation. Because of this, the total
amount of methanol converted per BAS increased strongly with
decreasing BAS density. However, it also caused lower rates of con-
version of the dimethyl ether intermediate to useful products. In
terms of the total amount of methanol converted per BAS to useful
products, mainly light olefins, the set of zeolites showed optimum
performance at intermediate BAS density (HZSM-5 severely
steamed at 450 °C with a BAS concentration of ꢀ0.2 mmol gꢁ1).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2013.07.021.
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